Yin Yoga is wonderful, but not necessarily the answer for every problem or for every body. If a student has laxness in the ligaments that could be contributing to instability in a joint, Yin Yoga may not be the best way to treat the condition. Generally where we have instability, we need to tighten up the area. Yang Yoga practices may work much better then because one of the jobs of our muscles is to contract and protect the joints.
Rather than have your student do more Yin Yoga in the pelvic region, you may want to advise her to work on strengthening the muscles there. Kegels exercises, core strengthening yoga poses and lower back work may be what she needs.
As always, she should check out this plan of therapy with her health care professional first and see if he or she agrees that this is safe to try. In The Complete Guide to Yin Yoga there is a section on postnatal recovery for women to help them regain their stability. You can read the post that that section came from here:
Postpartum Yoga
Please let us know how it goes!
Cheers
Bernie