Any one have it under control, return to bad foods successfully?

My son's symptoms have been under control for years and now he thinks he can return to eating fried food, onions and processed food because he feels so good. Nexium and a strict diet brought him this far. He had hernia surgery 4 weeks ago, lost weight, and think he needs to eat fatty food to gain the weight back. I am afraid it will be a disaster. Any suggestions?

Has anyone else returned to high fat and processed food successfully?