Thanks for sharing this story. Like you, I am sorry that your student had such a bad experience, and I can understand why she doesn't want to return to another Yin Yoga class. I can't say that I blame her, but neither would I blame you for her experience.
You are right; injuries can and do happen in yoga classes, and of course in any form of physical exercises. Sometime the injury is a result of the “last straw,... tissues become weak with repeated stresses and not enough recovery time and then one small, final stress is enough to trigger the injury. We humans love to look for the proximal cause of events and confuse those with the actual causes. It is easy to believe that “Because we were just in a yoga pose, it must be the pose that caused the injury., This attitude ignores all the prior, predisposing events that set up the conditions for the injury. I would love to hear from your student about all the other things going on in her life before the injury: what other physical activities had she been doing, had she experienced any little “tweaks, or pains before, how is the level of stress in general in her life. Also, I would have liked to asked her what she was feeling while she was in the pose. Were there any warning signs that maybe she was holding for too long or had gone a little too deep in the pose?
But, regardless of all that, injuries may still happen. It is rare, but not unheard of. It is more common for hypermobile students, but not for all hypermobile students. There are 3 general causes of hypermobility, which I describe in more detail in this article Hypermobility and Yoga
, (this is a revised version of the article which you may not have seen yet):
1. Enhanced ranges of motion due to an injury to a joint
2. Enhanced ranges of motion due to a connective disease or disorder
3. Enhanced ranges of motion due to skeleton variations.
Do you know the cause of your student's hypermobility? If it was one of the first 2 categories, Yin Yoga may not be a great practice for her. She would be better advised to work on strength than enhancing length. However, if she is hypermobile simply due to the shape of her bones, that is usually not an indication against Yin Yoga.
For hypermobile students in the first 2 categories, it is possible to gain benefits from a yin practice but they should 1) start with very short holds ~1 or 2 minutes and 2) only go halfway to their ultimate end range of motion. And, then, they should be taught how to attend; how to listen for signals from the body that may signify “enough for now!, And, coming out of the pose should be done carefully, with some prior muscular engagement to stiffen the joints before moving them. This is probably what she was referring to when she said, “she didn't engage the muscles properly and messed up both her shoulder joints/ligaments,. Yes, we do engage muscles to move the body and come out of postures, but there is a fragility that develops when we stress the tissues for a long time. Adding a bit more muscular engagement around a joint can help to stiffen it and protect it better. This is rarely a problem for normal people (even category 3 hypermobile students) but can be a challenge for category 1 and 2 students.
What to say to your student? That's a good question. Perhaps something along the lines of “I am sorry to hear that an injury prevented you from doing your yoga practice for so long, but I am happy to hear that you plan on coming back soon. I would be happy to talk to you about your experience and the possible causes of your injury. It could be a great learning opportunity., I can understand your fear of opening up some sort of liability for her injury, but it would be great to know the answers to the above questions and use it as a learning opportunity 1) for her to understand her body's signals better and 2) for you to learn how to cue students to better attend to their signals and guide them safely out of poses when these signals arise. Having said that, I don't believe that you are personally responsible for the injury, as you are not a therapist or doctor and your yoga class was not offered as therapy. Any physical activity has an inherent risk to it. It is unfortunate that this time, it triggered an injury for your student.
Let us know how it goes.