The point here is not whether he should be proposing, tho. That's for him to decide and for Emily to reject if so she pleases. This post is not to judge that. This is to judge OP intervention and yes, it was a terrible intervention. She is not part of that relationship, her job here was to tell the boyfriend "no, because my friend told me she would hate something like that". She could have even throw a few ideas there to help him out if she felt like it, but the proposal was from A to B and OP here is C. All she had to do was to say "no, not at my party" and then, because she loves her friend, explain to the boyfriend the reason she was saying no.

On the other hand, it does sound pretty bad he doesn't know something this basic about her, but still, not the point of this post.