I agree that Michael was an asshole, no doubt. But I also think Debbie was wrong too.
they decided, "if Michael thinks we're an embarassment, then let's be one!"
I can see the guys doing this, but I can't see Debbie doing it on purpose. Yet if she did, then why is she surprised when Michael acts out too? Her intention is what confuses me and I wonder how the topic of crashing the party was brought up and by whom. It was probably just a casual comment of, "Brian's giving a donation so we can all go to Michael's fancy party, yay!" At the end of the diner scene, she honestly seemed like she accepted Michael's desire to not have her at the party.