The main thing to figure out with any passive female student is WHY she is resistant to physical contact. Does she feel weak, powerless, ineffective? Does she "hate violence" or is she terrified? If it's simply the newness of being physical (some women have never hit anyone or anything in their lives), often times once she understands the mechanics of the techniques, she's able to have fun and hit harder. It's easier to encourage her to access her aggressiveness when she does not have as much emotional baggage to overcome. But for many women, especially those with histories of violence, rape or abuse, getting self defense training (i.e. learning how to hit or kick with power and commitment) is a HUGE step and it takes an enormous amount of courage to even walk through the door - even if the woman sincerely wants the training. So try to tune in to her situation - she may or may not tell you about an abuse history, but sometimes it's obvious - and then figure out how to best instill the mental and physical skills she needs that will push her beyond her comfort zone (where the real empowerment happens) without sending her running for the hills. If you can successfully help her break through these psychological barriers and access her fighting spirit, many times a woman with an assault histories end up being the MOST dedicated and determined and powerful student! I know this is self defense training, not therapy, but I'll tell you, good training can do FAR more than 10 years of therapy if it is presented in a supportive manner. I've seen it time and time again when women really embrace how powerful they are and that they have a right to be safe. For survivors, beyong learning physical skills, they also learn that whatever may have happened in their past is sure as hell not going to happen again without a real fight on their part and their lives are forever changed!
But how to do that? The first thing that comes to mind is that it would be very beneficial (I believe necessary) to have a confident, supportive woman help you teach female students. I know I talk about this a lot, but women are able to learn more effectively from a female teacher - especially timid, shy or passive women. It's just easier for a female student to accept messages like, "You DESERVE to to protect yourself! Your safety is up to you! You don't have to feel like a victim anymore. " etc. when they are coming from another woman. If you had a female counterpart, you could demonstrate the techniques or concepts together and then your partner could guide or "coach" your friend as she learns. The goal is to give your friend "permission" and encouragement to tap in to her fighting spirit (something that is VERY difficult for many women), as well as provide a model of what that looks like. By watching another woman do exactly what you are asking her to do, it may inspire her to push past her resistance and go for it! I know this to be true because after I demo a rock 'em, sock 'em fight in our Level 1 class, my new students frequenty say things like "When I saw you do it, even though I was scared I though maybe I would be able to do it as well" or "It seems easy for a big guy to defend himself, but you made it feel like we could all do it!" etc.
With passive women I usually use the "layered teaching" approach - not asking too much too soon or they mightt bolt. I start on mitts and pads and do the old "building on success" thing - when they strike weakly, I say "O.K., good! Now let's go a little harder... O.K., blast right through your target - I know you can hit/kick harder than that, so don't hold back!", etc. etc. I also move away from the student slightly with each strike (taking the pads with me) so they have to move toward me or close the distance to strike again. It's subtle, but it encodes a subliminal willingness to close in and finish a fight, I think. Also, doing a continuous knee strike drill (pads at groin level), where she must drive you back all the way across the room with strong knee strikes is a great one! As her skill increases, you can move around a lot more, forcing her to adjust. Also, go hi/low with targets - mix it up so she is thinking less about "Oh, I don't want to hurt anyone" and more about just finding and hitting the target. Eye strike-groin strike-palm strike to the face, etc.
Blocking drills are also effective because it encourages women to be proactive and GET THEIR HANDS UP - if they don't they will get hit and they don't want that to happen. The reason I think this is an especially effective building block for passive women is because they think of it as DEFENDING THEMSELVES rather than hurting someone else (it might take a while to wrap their mind around the idea of really harming another person - even for self preservation) and are usually more willing to engage physically for this purpose. If you swing relatively forcefully with big, haymaker-style punches, she will get more and more comfortable getting her arms up and inflicting a little pain with the blade edge of her forearm. We DO make contact with them if they fail to block effectively (not hard, just a tap to the side of the head) to show how important it is not to get hit. Once she gets comfortable with this drill, we show her that, because she already has her hands up to protect her head, it's not a big deal to deliver a strong open palm strike to the face from that position. Bingo!
There is another level of resistance that occurs when you practice against real male anatomy - some women recoil up at the idea of REALLY nailing a man in the testicles with her knee or shoving her fingers into his eye...but they MUST have this frame of reference for real world defense! Just training on mitts and pads is not enough. We use the same positive reinforcement concept when they start working against our armored instructors, i.e. "O.K., send your knee right up and under to his testicles. Good! (even if it's not) But you know what? You can knee him WAY harder! If you want to end a fight, you're going to have to REALLY strike hard!" The job of the male instructor is not to "give away" anything, so if she is not striking hard enough, he should not respond realistically until she does. Obviously, doing this safely requires adequate protective gear for the guys (duh!), but the results are very beneficial! When she understand HOW HARD she really needs to hit or kick to have any given target to an effect AND has a chance to do so ON the actual target, it helps her break through some of her resistance. Again, the female instructor needs to be right there to encourage her to strike harder than she wants to.
Another great way to get your friend to tap in to her fighting spirit is to do some verbal boundary setting drills where she must use her words and body language to control or de-escalate the situation. But, as you are already friends, it would work better is she was faced with a man she was not as familiar with. Again, your female co-teacher should be out on the mat with her (standing behind her) helping her find her words and maintain a safe distance, good footwork, etc. as she faces her verbal assailant. The "assailant" should wear sunglasses (maybe a hat of some type), so it is a little more impersonal for her and use his words and physical threats to help push her to REALLy step up and set a verbal boundary. The scenarios should simulate situations she is likely to find herself in - drunk guy, slimy, sexual predator, won't take "No" for an answer guy...etc.. Even though the scenarios are not physical, she should come away feeling some adrenal stress and perhaps more in touch with anger, rage, etc. that she can ultimately transfer into physical power!
As ever, more than you asked for...but my honest opinion is that in order to tap into a woman's "aggressiveness" or "fighting spirit" and break through resistance, you need to address both the physical as well as the psychological, emotional aspects. Please let me know if any of my ideas are helpful with her!