I do not like this kind of bet. A bet that no money nor anything is involved. I mean nothing is involved because one of you can decide not to fulfill the betting promise. Example is if your friend did not lose more weight and still decide not to pay for the meal? This can happen. I do not bet with people. I prefer to go to casino or bookie to bet instead.
You are fixing your mind on the wrong thing, the goal here isn't about getting money from each other but facilitating the journey of self improvement. Even if they don't end up fulfilling their side of the bargain there's no harm done because that challenge would make the individuals work on themselves. Asides from this I don't think it's a good idea to get involved with people in anything pertaining to gambling, because Humans act different when there's money involved in something. I also prefer to bet in casinos or bet centers instead of doing it with people but I love the idea of betting for self improvement.