I can imagine your student's frustration: so many people giving contradictory advice. I am not sure I should be adding to that list! Let's start with some basics though. If the ligaments supporting the sacrum have been weakened by being overly stretched, thus causing the sacroiliac joint to be hypermobile and unstable, there is no known way to help the ligament shorten faster. Over time these ligaments will slowly shrink. For this reason, I can see why the doctor is suggesting "do not stress that area! No doing postures that will stress these ligaments and prevent them from shrinking back to their original size., In this model, such advice makes sense.
On the other hand, many therapist suggest that when a ligament can't do its job, then we have to train and strengthen the muscles to take over. Strengthening the muscles may help to reduce hypermobility at the joint. But, stretching is not part of that equation: the intention is to stiffen the muscles crossing the pelvis. Walking can be part of this strengthening, as can some resistance training done with care and proper guidance (I would suggest
McGill's Big 3 as well as some squats. But, I do so guardedly as I do not know your student. These may be exactly the wrong things for her.)
Could Yin Yoga help? Maybe, but Yin Yoga will put a tensile stress into these ligaments which is what the doctor didn't want. However, many therapists believe that in order to strengthen a ligament or tendon you must stress it and the only way to do that is via a tensile load. In my own view, if you never stress the ligaments, they will atrophy-yes, they will shorten over time but with no stress they will also become thinner and weaker. Yin Yoga could be used therapeutically to put enough stress into the area to help stress the ligaments, but don't overdo it so that they also lengthen.
There is no formula that can be given because every body is different! Your student will have to find a therapist she trusts and can work with over time. Test a little, see how it goes. If it doesn't work, try something else. Start slow but stress is the key! No stress -> atrophy. Too much stress -> degeneration. (See my article on
Antifragility for the importance of stress.
Good luck finding that balance!
Cheers
Bernie