That can be a little tricky. How do we know it is FACET pain and not a strain, or arthritis or something else? Had he fractured it before? What is causing the pain/what is history/what makes the pain worse, etc etc? Has he had X-rays or MRIs to make sure its not stenosis or bulging disk, etc.? Because some conditions require flexion strengthening and some require extension strengthening.
i would ask for a little more info and decide based on his responses.
Check out my video on lower back stretches
Core strengthening is one of the top tools available for back pain in general. I think careful stretching is also helpful. But this is, as Tim is suggesting, kind of the middle of the equation.
It is probably helpful to get some background: Has he been to a physician? What diagnosis if any was made? Has he been through PT, and what did the therapist recommend? Is he currently on any medicines? Is he currently being treated? This kind of information will let you proceed from a strong beginning.
It is also probably (generally always good, but here especially so) to do some static postural alignment assessment. Then decide what needs focus for stretching, in conjunction with the core work.
I use a vinyasa I designed I call ‘spinal series’. It isn’t some magic thing… you are a 200 hour RYT yes? so I think using the 6 movements of the spine, and working outward from that, paying attention to mulha bandha, might be a good start, just to keep the spine flexible, but always reminding him to stretch in a line of energy, rather than to a preordained number of inches, or degrees of motion… never crossing over to pain. I would also caution about seated unsupported forward bends, unless his medical provider has clearly stated those would be good to do.
I would like to recommend to you the work of Gary Kraftsow. I have had a workshop with him and find him to be a really top notch teacher of yoga therapy. He founded the school of Viniyoga and trained with Krishnamacharya himself. He has several books out which you might find helpful in this type of work.
I am not at that level, but I did do a couple of blogs on the spine/backbending and on spinal twisting if you have an interest.