But, if OP actually told SO that she's rather he keep his commitment to her, I can see SO telling his folds that OP won't let him cancel, thus putting OP in a negative light to her (maybe) future in-laws and to the family friends. To me, the SO put OP in a no-win situation, and it my DH/SO/BF did the same to me, I'd also tell him the choice was his, because he should be the one to take responsibility for his decisions and suffer any consequences of those decisions. It wouldn't be said in a PA manner, but it might make me rethink the relationship. Regardless, someone is going to be disappointed-OP or SO's parents & friends.
(Disclaimer-my personal experiences with both my IL's and my parents could be coloring my thoughts more to the negative side of things.)
This has been my experience. My DH was worth training out of the bad habits taught to him by his parents
I have to admit that I spent years waiting for everyone to grow up, thing was, DH had never been taught that he was equal as an adult, to his parents. Everything came together for me when I was 30, pregnant with my second child, MIL once had been behaving badly and instead of apologizing, said "You just have to over look that, I have a hard time remembering that you guys are young adults". I figured that if I was a 'young' adult at that time, I would never be an adult.
The lesson that I learned the hard way? Sometimes people need to be given the script, once they are able to see past their parents ideas of how things are then they will be able to act reasonably.
OP- when I was in your situation I would have reacted exactly the same way you did. Looking back, I wish I had been more proactive, it might have saved me a lot of wasted years. Try saying to your BF, "why don't you spend the beginning of the evening with your friends, then come over for a late dinner and we will keep our plans for tomorrow."
I do think that casual plans can be changed for something more formal or business, I don't think this is the case. In my family, friends coming over for dinner is fairly casual and relaxed. If these were guests coming from out of town for a dinner party, then the parents should have told their children as soon as plans were made.