I can well believe that after years of repeatedly stressing her lower back that your student now has issues. Prof Stuart MGill discusses these types of lower back problems often in his research, books and DVDs. I can give you some of his thoughts, but I am not a doctor and can't prescribe what your student "should" do. You may want to check out McGill's site
and books yourself to learn more.
Soft tissue damage is hard to detect, but pain is usually a good sign that there is something wrong. Healing can be accelerated by mobilizing energy through and to the area but the trick is to find out how much movement is best. Too little and no healing occurs, too much and the area just becomes more damaged. McGill's findings suggest that we can mobilize and strengthen the lower back area if we do it with the spine remaining in neutral. He recommends poses like plank or crocodile to strengthen the core muscles on all sides of the lumbar, while keeping the spine "straight" or neutral. I have found crocodile and the side versions to be very good. Balancing Cat (he calls it Bird Dog) can also be very effective.
Now both of these are yang exercises. In Yin Yoga we are trying to strengthen the deeper tissues. In yang yoga we are targeting the muscles. I think your student should employ both modalities but she needs to go slowly and make sure her own doctors are onside with the plan.
You have already prescribed some yoga practice for her and that's great. I would hope that in her restorative yoga there is some gentle yang practices as I described above (and these can be very gentle! A balancing cat and crocodile with the knees on the floor should be doable by most students.) If back bends are painful, then you are right to avoid them, for now. And that brings us to the question of counterposes.
If your student can't do a back bend, what counterpose should she do after forward folds? Well, that can depend on what type of forward fold she did. For example, for some people doing Butterfly what I see if the upper back in extension and not the lumbar spine. For them a counterpose like Anahatasana
may be all they need. For really flexible students, after a fold like Straddle where they just flop down on the floor, again their lumbar is not flexed but rather their hips are in flexion, so some hips extension may be what they need: Swan or even Camel for the very flexible, may be good.
However, lets assume your student is doing something like a Butterfly and her lumber is flexed and she is feeling it there. How to give her some relief? If you can't do any extension of the spine, try twists. Twisting is a counterpose for almost any spine movement. The idea of the counterpose is just to get energy flowing again to the area just worked. While counterposes are desirable, they should never be as deep as the initial pose.
Another idea: you said that lying on her back is painful - but can she lay on her stomach? If so, that is a gentle backbend position for the lumbar. Maybe that could work for her. And if that works, does she have the hip flexibility to do Sleeping Swan? That could be another possibility that she can work up to (but not quite yet because getting into it might be challenging.)
I hope these ideas help. Let us know how it goes.