Logically, someone else would do it if he loses and chooses not to go, since it's essentially the same situation as winning.Doesn't sound like predetermination if he can lose and then simply choose not to go, resulting in nobody going then.
He will also lose the vitality reward promised by the system for beating Irina. I hope he knows what he's planning to do. But then again, he doesn't seem to care at all about his lifespan, so it might be irrelevant for him anyway.
The raid happens because the nobles are feeling like they need to punish the commoners for getting too uppity, but I don't believe it ever said what would happen if Frey just chose not to go.
If anything, the best idea would probably be to win and pretend to be the raider anyway, but the issue with that would be that you probably just end up with two raiders instead.