Tell your friend to make slow changes in moderation ....
I am still a fat fvck, but I'm down a couple of pounds over the last month or so just from cutting out most of my pop, and not ordering appetizers with my entree's when going out for lunch or dinner.
The changes I have made were easy changes that I could live with.
I save a bit of cash, I haven't cut down on the things I really enjoy (good beer, Steak, Pork, etc.)
Because the intake reduction is a small percentage, (read about 5-10% of total calories) and because I also drink more water than I used to (helps to trick myself into thinking I'm full), I still eat a good amount more than most people, however. I eat less than I used to, but not enough that my body has slowed the metabolism. In a month or two more I plan on cutting back by another 5 to 10% .... Then, maybe eventually after a year or two ... I'll be down to about 250 pounds ... (I weigh about 275 now)
I don't consider what I am trying to do a "diet." It's really just a "kernel hack."
The Hackers Diet
It's a SLOW process
But it can be done ....
EDIT: Actually ... now that I think of it .... at lunch I used to drink at least 2 or 3 cans worth of pop, probably I've actually cut down by 10% to 15% ... just by cutting out pop and appetizers ...