Gaining

I concur, if fast food isn't a normal part of their diet, they'll feel like crap binging on it - I know I always do. And I also agree with georgejorge: introduce the super-fattening stuff gradually, increase portions gradually.

It could also be something in the food, like a lactose intolerance, gallbladder issue, etc.

Happy gaining! Your feedee sure is lucky!

Eating full, yet balanced meals is the way to go. And sub in heavy cream for milk. Good stuff!