I have little doubt that most diets are completely unsuccessful after 5 years, but I also wouldn't really expect a doctor to have so much information on various diets. Your friend would probably have better luck asking a registered dietician for that information.
Anecdotally, most of the weight loss success stories that I've heard involve the person making lifestyle changes that naturally appeal to the person, and also just happen to involve increased physical activity and/or caloric restriction. So it's not a one-size-fits-all type of thing. It's more like sitting down and saying to yourself, "Self, what could I change about my life that would primarily be really fun, but would also promote weight loss?"
For me, I'm an economist by training, so I found enjoyment making a satiety vs. calories maximization problem out of it (i.e. what foods kept the hunger away for the longest time for the least amount of calories consumed?). So, I dutifully logged everything that I ate or drank in myfitnesspal and kept notes on how long everything kept me sated. Now, I have a list of foods that, for me anyway, keep the hunger at bay so I can simply eat less food and not feel tired/hungry/grouchy.
I realize that most people would hate doing this, but since I have a love for The Dismal Science, it appealed to me. Others needs to figure out what would excite them.