Breaking a Friends Engagement

My friend just got engaged to someone I think he should break up with. All the times I’ve hung out with them, this guy’s given me the creeps. I think he’s weird and not very nice and a total step backwards for him.  I never said anything about it to him when they were just dating. But now that it’s so serious, I feel like I HAVE to do something. How do I bring this up to him? If this goes badly and they have to divorce, I would feel terrible to have just stood by and let it happen.