Even assuming both were willing and wanting to do it a fight at the actual reunion seems like a really bad idea.
1.) The chance of someone getting hurt increase when you don't have a well defined rule set, paramaters, proper fighting space, etc. This is why all legal fighting is done in a ring, cage, etc with very specific rules.
2.) This is going to cause major drama for everyone involved as everyone will be able to witness it. I'm not just talking about winning vs losing here either. The entire fact that a fight happened between the two of them at a high school reunion is all everyone that hears about it is going to be able to think of when they talk see you guys. The social ramifications are major.
3.) Cops are probably going to be called/involved.
4.) Stemming from point #3. The fight isn't likely to even prove anything or finish. It's going to get broken up by spectators and/or cops.
Fantasy is one thing but their is a reason why stuff like this doesn't happen in the real world often.
If they're really that insistent upon doing it then a private setting with rules everyone is comfortable with is a much better option, albeit there are still risks due the very nature of what's going to transpire.
