My Friend Has Had Mostly Constant Diarrhea For Years. His

My friend has had mostly constant diarrhea for years. His Doctor has lab tested his stool and it's fine. I gave him some refrigerated probiotics and he said he occasionally did not have diarrhea for maybe a day. Someone suggested he try a food elimination diet to look for food allergies that could be irritating his colon and intestines. I don't know what that diet is? Any suggestions?

Sincerely, Cheryl