Is it so wrong for him to prioritize a prior commitment? 

When you’ve been dating someone for two months, you might not feel ready to make huge plans with them. You might be comfortable planning the next couple of weeks together, but planning a big vacation only two months into dating could be kind of catastrophic. If you break up, someone is going to have to pick up the bill, and if you’re a cheapskate like me, the airline will probably be unsympathetic because you are flying Basic Economy and didn’t opt to protect your trip. So really, you will probably be picking up the bill. 

Early in a relationship, you can’t always expect your partner to put you above their friends, nor should they. I’m not saying that you should have boys’ night six times a week, sidelining your girlfriend and barely seeing her. But it’s not like you should never hang out with your friends again after you get together with someone! After all, your friends have been there for you since long before the girlfriend came into the picture, and they’ll be with you long after she’s gone. You shouldn’t abandon your buddies under any circumstances.

Even though they’ve only been together for two months, the girlfriend in this story really wants her boyfriend to come to her graduation. That would be all well and good, but he’s going to be out of town on a once-in-a-lifetime vacation with his bros and his family, and he doesn’t want to miss out on that.