Don't take things too seriously and don't worry about what her friend did, it was actually not a big deals. Getting mad for stupid things is very immature and girls don't like that. About her leaving you well just get over it, if she likes you she would come back, if she don't like you then you'll wish she never came back because it would have not worked out. Just ignore her and act like if you don't know her, that will make her realize and think you are over with her so it will make her want to get close to you if she still interested.