I have only a few worlds of advice for you: STAY OUT OF IT.
If George is seriously thinking about hooking up with an unsuitable mate just because she might buy him stuff, then there's going to be problems down the line. If you tell him to do it, he'll blame you later when it goes bad. If you tell him not to, he'll ignore you or turn angry (defensiveness). The best you can do as a good friend is to be there to listen and give emotional support.
If Susan and her friends tend to ditch their friends each time they get a new boyfriend/girlfriend, that says something about what kind of people they are. I vote you look for friends somewhere else.