Michelle, I think you mean L5/S1 as that is where most low back injuries occur and L1 far from the S1 joint. From what you say it sounds like she has osteoarthritis of the L5/S1 joint. Either way, I’ll assume your client is a beginner. If this is the case, this is what I suggest:
1. warm up of about 5-10 minutes
2. circuit training consisting of machines. do only 1 exercise per body part using a light resistance that she can lift for between 12-20 reps.
3. Increase the reps she can do before you increase the weight she lifts.
4. I know you know this but if it hurts, dont do it.
5. No need to do multiple sets in a beginner as this will increase DOMS.
6. Show her that she can exercise safely. Keeping her working out for the long haul without injury is a key to keeping her healthy and hopefully reducing any pain she may be experiencing.
I hope some of this helps Michelle