I assume that you have medical clearance from the doctor.
You do not say how long ago this happened but there is reason to assume that this person has worked with a physical therapist after the surgery. Ask the client for copies of the exercises he did in PT, and maybe even try to contact the physical therapist for more information.
In cases like that, I start with my own fitness assessment and usually share the results (with client’s permission) with the PT. Then I incorporate the PT exercises in the initial programming until I have a clearer picture of the client’s abilities.
One thing I stay away from with everybody with ANY lower back problems is unsupported forward flexion (example: having people bend over at the waist and do two arm dumbbell rows or triceps kickbacks).
Hope this helps.
Before you start training the client, find out for sure which lumbar vertebrae have been fused.
In your post you put L3-L6. FYI, there are only 5 lumbar vertebrae.
After you find out, I believe it is important for you to understand the function of the entire vertebral column, but more so the limitations your client will have as a consequence of spinal fusion.
Too, don’t forget the muscles that originate and attach on the lumbar spine. You will have to take this into consideration when designing the program for your client.
I hope this gives you a bit of direction.
You need the doctor’s clearance; hopefully the person finished physical therapy and can tell you what they did there. By L6 do you mean into the sacrum? Yes, there are only L1-L5.
You also want to stay away from lifting both legs simultaneously. With perfect form, try the plank.
Be careful to take things slowly; starting from the beginning.
I am sorry to state this and I mean no ill intent. But if your asking the questions you may not be the best option (at this time) for your client. Finding and establishing a relationship with a qualified practitioner in your area is the best advice I can provide.
I noticed you’re in Ohio – try contacting Greg Mack in Columbus. He is one of the most detailed practitioners in the fitness industry. Here is his contact information http://www.physiciansfitness.com/pfnet/staff.aspx