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.
Actually they're catching fun, they are not betting, if they serious in betting they most be a deposit from it, but I know that if seriousness comes in, I believe that they will come with deposit, what you said make sense, because one person can decide not to abide on their existing agreement, so what they needed to have a third party or a central place to deposit, so that anyone that win the third party will give it to the person, I can only participate in this kind of gambling when I know that is a just a play.