Ornish was tough. My FIL tried to stick with it some years ago. Couldn’t do it. There were a lot of Tex Mex inspired dishes, ironically.
OP, how big a change is this is your friends’ lives? If they used to eat a lot of fried foods and fast food, just making some directional progress toward a better diet would be great. If they could eat baked potatoes instead of french fries, for example, that would be great. Jumping all the way to veganism would be too much!