I know this hurts, and god knows you don’t have to be happy for them, but being angry/upset doesn’t really help you either. They’re getting married. It’s the reality. And as much as it may be tempting to blame her and her behavior, HE made the choice to leave and HE made the choice to propose.
Honestly? Your best bet is to wash your hands of the situation and try to move on. I know it hurts, little dove. I promise it won’t hurt forever.