In my experience mixing friendship and work is a bad idea. It's hard enough to be cofounders with a friend. I won't recommend adding a power imbalance to that formula.
Not that you’d ever know of course.
I don't know about that. Sure, they might not have been totally happy with it, but if they're still friends with them, clearly they didn't hold any grudged. I think it's reasonable to give someone the benefit of the doubt that they aren't just being oblivious to the fact that they've been ghosted or something unless you have evidence to the contrary.
Similarly, individuals can judge success by their one sided satisfaction with the result. e.g. "I am still very happy with the relationship" = success.