You know, if I were that guy, and if I liked you enough (even just as a friend) I'd be trying to invite you along to my buddy's party. Doesn't exactly matter where the date is, it's that I'd still try to spend lots of time with you.
If that couldn't work, I'd try to do both, and explain the situation.
I'd still try to make it up and have a special date, of course, just us two (so I'd be making a counter offer to show that I'm still interested).
You could give him the benefit of the doubt, but be cautious. This is showing "flaky" behavior at the very least; and you don't want to date a flaky person. Yes, everyone screws up (unlikely something like his friend's birthday party would be forgotten about, but it's possible), so you trust him on this one. But, you don't rush to be with him. Keep showing interest, but don't jump through hoops.
If he flakes again, though, forget him and move on.