How strange to think that yoga could tighten just one iliopsoas and not the other? Maybe, but her yang yoga practice would have had to have been very asymmetric and surely her teachers would have noticed. If all the yoga she had been doing was Yin Yoga, it is very doubtful that yoga would have caused her iliopsoas to shorten - Yin Yoga doesn't target that muscle. In any case, her real concern is the pain she is experiencing in her back, so let's look at that.
If her pain is being caused by a short iliopsoas, which in turn pulled the lumber vertebra out of alignment, then it seems obvious that her therapy should be focused on lengthening the psoas and realigning the vertebrae. Yin Yoga poses like Saddle can help to release and lengthen the iliopsoas but it also arches the spine which may not be a good idea for her. Dragon could stretch the psoas without moving the spine. She could try those and see how she feels, but since she is under medical care now, I would only suggest these so that she can talk it over with her doctor and see what he thinks.
There are many causes of back pain, so again, i will not presume to remotely diagnose your student. Spondylolisthesis has many causes and I do know of one student who suffers from this because of her career as a gymnast. But, is this really your student's problem? It would be easy enough to find out: an Xray or MRI will tell her. If this is the case, the next step is to figure out why it happened and then work out a therapy for her based on how advanced it is. Strengthening the lower back and some physical therapy may be useful, but she would want to avoid extension of the spine, especially in a Yin Yoga manner.
I would encourage you to read this post
on spondylolisthesis specifically. This longer post
may also be helpful.
Finally, any doctor or health care provider is just one part of your student's advisory team, just as you are. In the end, she is the one flying her plane and she needs to take full responsibility for getting better. She should listen carefully to her doctors, etc, but if she doesn't feel comfortable with what she is hearing, she should seek other opinions. A good resource for people with lower back disorders is the work of Dr Stuart McGill
and his books and DVDs. They are a bit technical, but not beyond the ability of most people to understand.