Thanks for raising this point: it has been
known for twenty years that some antibiotics like ciprofloxacin can weaken collagen. I am sorry to hear that your student is suffering from this effect. (You have motivated me to write
an article on this topic, as it is probably not widely known that some antibiotics can weaken fascia and lead to
tendon rupturing and other problems.) Given that your student has suffered this, your question is — how can she regain her lost flexibility?
I do believe yin yoga can help, but I cannot cite any specific studies that show yin yoga will reverse the effects of antibiotics (because I don't believe this has been studied). I can only offer this hypothesis: since some antibiotics act to degrade the collagen in ligaments and tendons, stresses like those created in a yin yoga practice which stimulate collagen deposition should help to rebuild these structures.
For your student, a generalized yin yoga practice should be fine, as long as you teach her to pay attention to any signals the body may give her that she is staying too long in a pose, or going too far. She should be alert to any tweaks or little pains while she is in the pose, when she comes out of the pose or over the next day or two. If these do arise, she will need to back off in both time and depth of pose. As long as she is not experiencing any pain, she shouldn't be in any danger. Remember, we play the edges in yin yoga: there is no need to go to a maximum depth in a pose. Time is important than intensity. By avoiding really deep positions, she should be quite safe.
Good luck and let us know how it goes!
Bernie