The first thing I would ask is whether or not your client is medically cleared to begin an
exercise program (what kind of surgery?). Plus, has any physical therapy been completed. (if so , I would connect with the PT as well for any special considerations). Then you can begin a slow and progressive plan for your client based on goals.
As far as the weight gain, the training will only supplement a healthy diet. The combination of the two will help your client lose the weight, but progress slowly, and refer your client to a Registered Dietician if necessary.