Could you be smooth and cool enough to shake his end and then "serve him" with well put together list of items she owes you for in a crisp, white envelope. He'll ask "what's that" and you'll say "your girlfriends debts that I paid off, those are now your responsibility", then you'll pad him on the shoulder and say "read it at home then let me know how quickly you can pay this sum off and in how many instalments, my email is on the bottom of the letter". Then walk away. Any time he'll try to come back to you during the event and discuss it you'll just keep repeating "look (his name), this is probably not the best time and place to be discussing this, take it home, read it, then let me know". If he gets shouty at any point you'll reply "Listen (his name), she's your fiancee and your future wife, not only you can't expect me to pay your bills for you, but any discussion about it is a bit cheeky on your part, don't you think? If you don't like the responsibilities that come with this relationship, pass them onto another guy or something" Then walk away.
They'll never pay you but they'll always make sure not to be in the same room as you, thus saving you any future heart aches.