As a father, I don't understand parents who put their romantic relationships ahead of their trust in and emotional well being of their own children.

You need to start secretly recording what's going on, otherwise your mom isn't going to believe you, sadly. I mean you'd think she'd believe her own child over a boyfriend. Good luck :(

Let your mom have her party and her fun, then a day or two later pull her aside and explain what happened to her. By then, the emotions of the day will have passed and more than likely at least one of her friends might have at least hinted at what actually happened. If when explained, calmly and rationally, she believes her boyfriend over you, you may have decide what kind of relationship you are willing to have with her.

YOU know you did this for your mom. That's all that matters. Your act of kindness says a lot.

How awful. :( I will never understand parents who choose to put others like that before their own child.

And I thought MY dad's girlfriend was a douchebag. OP, in your place I would seriously do something about it

How did you invite people? Emails or texts would be all the proof you need. First try just telling her when you're on your own with proof ready if she doesn't believe you.

Explain what happened

I'd take her aside and explain it to her. While you're at it, if she doesn't already know then tell her about how you and her boyfriend don't get along.

To be honest they sound perfectly suited for each other. The best thing you could do would be to let them get married and make each other miserable.