It's not about doing it on purpose or not
In the first place, I can't imagine forgetting my partner's birthday, especially if my partner is someone I've been friends with for years
But let's say, for argumentation sake, that I did: I still wouldn't forget plans I made not even a week before
And if I did forget and plan something at the same time, I'd apologise to the other person and ask to reschedule/say I'm not going
And if for some reason I didn't do that (and I fail to understand why I wouldn't), I'd be too busy blaming myself and 100% would not resent my partners for celebrating without me
It would've been different had they found a third person to replace him with, but they didn't

I still don't feel bad for him
"He broke a major rule in the relationship!" as if dude didn't literally forget his partner's birthday AND was too chicken to cancel the secondary plans OR leave early
It sucks but he's not the victim