Are you describing me?
I have a fwe more disks herniated, and have had surgery, but I did well in the gym until Christmas break.(Now I'm being lazy)
If he feels any pain at all he should stop. I have to be VERY careful in the gym, and lift light with high reps. It took some experimenting to find out what I could and couldn't do. There are quite a number of things I can't do anymore, like squating for one. Technique is vital for me to be safe. If I feel anything at all I stop and reassess the exercise. If I can't find a way to do it pain free I do something else. A lot of the stuff I do doesn't involve weights at all. Dips, pullups, crunches(on the inflatable ball), and so on are great, and don't put pressure on my back.
The exercise has really reduced my back pain. I have pain all the time, sometimes worse than others, but if I do rehab style lifting it helps reduce the pain.
Feel free to IM me if you have any questions or concerns.