I'm not comfortable about my girlfriend inviting her friend to a party. She rebounded with a guy while we were on a break.
Did he convince her to rebound?
Yes, he told her to do it often
You should not try to control your girlfriend, especially if she's on a break. If she is okay with exploring her options on a break from you, you should just make her an ex and move on.
She probably slept with him, too.
Even if that's the case, it is none of your business when you both were on a break. She had the right to explore other options just like you did during the break.