you will need a physician’s approval.
The diet part does not have to concern you. People are closely monitored diet-wise after that surgery and get their recommendations from their team of doctors and dietitians. What you have to watch out for is hydration. The stomach holds only little content, and one big gulp of water can easily make the person sick.
Chances are that you get a doctor’s okay but you may have restrictions depending on the health status of the person. An assessment should be done as on any other client. The person is probably very deconditioned, and you can train as you would otherwise and gradually build up with a focus on cardiovascular programs.
When I trained a client after that surgery, we basically stayed within the stabilization and muscular endurance phase of the OPT model. The choice of exercises is determined by the results of the assessment.
Hope this helps.