The answers given are all excellent. Clearance from the client’s physician and PT, and consulting with them in designing the safest and most effective program for your client is crucial.
I also was impressed with Miraslava’s thoughts and ideas regarding working with the Swiss ball. She gave some excellent exercise ideas. As long as your client’s PT approves, give it a try. I know the results for many orthopedic patients, including those with low back issues, have proved to be excellent.
I am extremely conservative as well. I therefore only want to add that initially I’d only do flat vs rounded back exercises. Of course, flexibility is important for recovery and a healthy spine, but until he builds post surgery strength, I wouldn’t have him do any exercises in flexion. I’d also wait a bit before very slowly working on rotation. I liked Miroslava’s suggestions. My best!
A few things to add.
It is very important that people with back issues avoid long periods of sitting. If their job is at a desk, they would benefit greatly from a treadmill desk or a desk that could go from sit down to standing heights. And sitting on a fit ball and then their regular chair in turns will also help. And getting up and at minimum pacing if a treadmill desk is unavailabe (and if they have a TD) frequently is good.
Stretching is absolutely necessary. A good general series is advisable, but specific stretches for the major culprits (hams, hip flexors, quads, spinal rotators/flexors/extensors, etc.) are most important.
Myofascial release can be amazing.
Work on posture and stability (what most are calling core) is absolutely necessary.
Aquatic exercise is very helpful.
Each client will have similarities and differences. It takes a bit of detective work to get all the answers, but most can be helped.