This problem gets a whole lot smaller when we now know that evil Greg wanted an intimate dinner with your parents in lieu of inviting them to this big party.
Could his side of this have been “Look the party is getting way out of hand, and your parents probably won’t enjoy it anyway, let’s just have a nice dinner with them before the party instead.” That’s not an unreasonable position to take, assuming that Liz is being truthful, and there’s no evidence that she’s lying to cover for him.
There was a lot of non-communication going on and caused everyone to think that Greg hates your parents and doesn’t want them ruining his party. The offer of a dinner seems to contradict the idea that Greg doesn’t want to have your parents around. The fact that the dinner didn’t happen is a bummer, to be sure, but don’t assign bad intentions when simple bad decisions fit so nicely in place.