When I first had my gallbladder out, I was told I couldn't eat high fat so I switched diets and was pretty good with my D. I don't recall having it bad. Over time I found I can tolerate the fat and that has not affected me either.
Without the gallbladder, the body can't process fat as much which is what would cause the D. Everyone is different so he should go easy and experiment first. He shouldn't worry right now..just relax and heal up from the surgery.