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.