My philosophy: things have to end pretty bad to lose a friend in a relationship. They would have to do something that showed me they were not at all the kind of person I thought they were; cheating would be a good example, so would not being willing to admit they might be wrong, etc...
Other than that, if things didn't turn out happily ever after, that's okay. Unless both of you are attracted to each other emotionally, physically and on any other level, then it's hard to grow together and make each other better people. If they found someone who could do that for them, I could only be happy for both of them -- he's not a bad person and deserves to find that someone and so do you.
I'm friends with my ex and we both know that we're not the right match for a great, long lived romantic relationship. But we do know that we'd be happy for each other when the guy who was just right came along, and that has created a sense of trust and a safe, honest source of advice.
As for moving in that quickly, there are no rules. My Dad and my Mum were engaged after 3 weeks. They were together for 30 years before Mum passed, but the chemistry was just right and they would have kept going. She always joked that "marrying her was the easiest fuckin' decision he's every made. He spent more time picking his outfit this morning." hahaha, so don't worry, anything can happen at any time -- whatever you do, be happy for him. His search is as hard as yours and as any ones, so worst case scenario he drifts away....best case, you have a good friend you can go to.