A lot of the answers you’ve received so far are top notch and great suggestions. As well as decreasing ROM, you can try doing isometric work. Take the movement out of it. Try wall sits instead of squats or have her hold a sumo squat position. You can do the same with the lunge position. Once she can hold these positions isometrically, she can move on to movement. This can also be a great time to cue the group on how to work within their own ROM and within their own ability rather than singling her out.
I hope one of these suggestions everyone has offered works for you.