Portion control- they would still be able to eat the foods that they like just in a smaller portion.
Gym membership- they could get a gym membership and work out a couple of times in a week, exercise is key to weight loss.
Seek a nutritionist- a nutritionist is very knowledgeable when it comes to nutrition so they would be able to help the friend out so they know what foods they can eat and what foods they should stay away from.