I recall the Jackie Gleason diet. It was based on monotony. You could eat anything you wanted but only on the day designated. IE, let's say you liked steak, potatoes, and pizza. On Monday you could only eat steak. Nothing else. On Tuesday you could only eat potatoes.,etc. The monotony caused loss of appetite. Jackie lost weight ...died at 71, not that unusual for a male in those days.
I'll tell you my Jackie Gleason story. When he was in BR filming The Toy he came into Jack Sabins with his entourage one night. I was dining with my parents who were regulars as we're most everyone else in the dining room. He was loud, obnoxious, drunk, every other word out of his and his groups mouth was frick and smoking a cigar in the dining room clearly to everyone's dismay and discomfort.
Joe Distefano, the owner, politely asked him to tone it down and would he mind not smoking the cigar. People still smoked in restaurants but that's the only time I remember anyone smoking a cigar. He went bat shite saying he could buy the joint, didn't care what anyone thought and do you know who the frick I am?
Joe listened quietly then said, do you know who I am? I'm the owner of this place and these people dining here are the ones who come to my restaurant on a regular basis, are my friends and my supporters. They don't want you here so I'm going to tell you to GET THE frick OUT OF MY RESTAURANT!
The entire dining room clapped and heckled Jackass Gleason all the way to the door.
To the OP why worry about your friends diet? Dude likes beer and bread. There are worse things in life he could like. Jambalaya with tomatos, Hot Pockets, Natchitoches Red Beans, Johnny's Pizza et al.