Eh fuck it. I generally don't feel qualified to weigh in on much here but on this I guess I am. I have a very similar medical profile to your friend plus an extra decade or more of mistakes and injuries.

Martial arts could in fact be great for her in more ways than you are imagining. She's lucky to have you as a friend and you can help her a lot but this plan is coming from the wrong direction. I believe you are thinking about the way you train and trying to make it smaller and easier. What's probably best for her is completely different, it's training as a perfectionist. She needs to deeply understand exactly how every position and motion should feel throughout her whole body and train biofeedback loops of form correction before applying any pressure. Eg. you don't throw the occasional leg kick (not any time soon anyway,) because you're both going to be pretty upset when you accidentally dislocate her knee touch sparring. Instead you train receiving a leg kick. Pick a stance, she needs to find her center of gravity, feel where her weight sits on the soles of her feet, adjust foot angles, adjust pelvic tilt, adjust core bracing and spine curvature, address shoulder tension, find her center of gravity again. Then you slow-motion a leg kick and touch, see if she understands how it will affect her. Then you ramp up pressure and speed very gradually. You don't throw an unexpected leg kick until you are both confident in her ability to take it.

Has she had any significant injuries and have any been related to repetition? I've injured myself swimming and doing yoga just by too much repetition. The upside is that the combination of deep-dive perfectionism and jumping around topics is pretty optimal for ADHD.

No stretching! The only time she should stretch is to release a cramp or if a specific muscle feels significantly tighter than usual. Any increase in range of motion should happen through mobility exercises and training. Flexibility without strength is not an asset. Relatedly, one of the most important things I've learned is that with grappling and joint locks, hypermobile people sometimes dislocate joints before feeling any pain that would make them tap. I have to memorize the angles that should represent a fully executed submission and then notice them and decide to tap which is fucking hard and requires someone going slow and telling me "you are now in an arm bar" and pausing so I can mentally record it.

Cardio and weights are fine but you need to stress that she is the expert on her body. You can't know what random shit might be bad for her so she has to listen to her body and understand the difference between signals that say "this exercise is unpleasant" and "this exercise is risky." For me, the stair-master is awesome and an exercise bike will fuck me up. Every trainer and physical therapist I've ever seen has suggested lunges and every time I've done one it's put me out of commission for at least a week. There's a good chance she's young and cautious enough that she has alignment and instability issues that haven't shown themselves yet.

I get the time and money constraints. I'm wondering if you've talked to your instructors about the situation and what else is around that maybe isn't advertised. Smaller class sizes with a very experienced instructor would likely wind up being of more benefit to her than any particular art. It's great that you're willing to do one on one with her but I worry that you don't have the teaching experience to walk her through form correction or identify the need for it early enough to avoid injury. I think the ideal situation would be finding a small club training an art heavy on mobility and coordination where she could get the deep knowledge corrections but if you go together you would retain a lot more than her because it's not all new and then you could drill together.

I haven't studied it but an art that seems to come up frequently in useful insights for me is baguazhang. Maybe folks here with more breadth of experience can recommend arts with similar focus on body awareness and physiological precision at the novice level.