OP, trying to control your girlfriend's choice of friends was a poor choice. A better option would have been to "hang out" with both she and her new "friend" despite whatever her friend might have done to your own. Afterward, you could have mentioned to her (in private) what transpired between her friend and yours. You could also have heard how she felt about the situation. Nowhere in your post do you even seem remotely concerned for what she felt or why she made friends with this new person.
As for what you should do? Learn from your mistake, for starters. Ask her how she feels about getting back together. Listen to her response and take her feelings into consideration. If this leads the two of you toward getting back together, then wonderful. If it doesn't, then perhaps this was not meant to be?
I only offer this advice because you asked for help.
Good luck with what happens,
OP.