Most Helpful Guy
When I got divorced years ago I learned (through friends and a councillor) that we can't make ourselves responsible for other people.
You are not responsible for how your guy friend feels, ever. Even if you were leading him on with the purpose of hurting him, which I assume you weren't. You're not responsible because you're not him. You're you. You're only responsible for how YOU feel. It's our ego's that tricks us into becoming responsible for everyone else that we love and care for.
It sucks that his feelings are hurt. But that's for him to resolve.
If he's so uncalibrated that he's going to ask you to marry him, then the more hurt her is the better. Obviously he was walking around without any real perspective on your current relationship and he needs to open his eyes.
His pain is not your fault, and it's not your job to fix.
The reality is that you need to really give your guy friend more boundaries, and most likely more distance.
It's possible, and this might sound ridiculous, but it's possible that if he's this uncalibrated then he might be dangerous. So keep your eyes open for stalking behaviour.
If I was him I would feel embarrassed and proud of myself at the same time. Proud that I took the chance to ask, but embarrassed that I didn't know any better. And I would take this time to really look at myself and my life to figure out why I was so delusional. At no time would I blame the girl for not wanting me back. I wouldn't take it personally. But I'm balanced and healthy.
I hope this helps?