It’s possible she may have strained one of the hip muscles at some point, or possibly have a bulging disc in her lower back, which is pressing on a nerve. Something similar happened to me in class, I came out of camel too fast, pulled something and then felt like I couldn’t move during savasana. Quite a scary experience. Unfortunately, it now seems to occur at random intervals.
I am seeing my osteopath in a couple of weeks and will ask him if he can shed any further light on the situation.