Most Helpful Girl
If I didn't know my guy was going to a party and he took a pic with a girl at said party that she posted, I'd probably feel a bit upset. It would just make me feel like an idiot who doesn't know where or what her man could be doing behind her back. Wouldn't you feel the same in reverse if she was at a party you didn't know she was at and posted a picture with another guy? I don't know that it would constitute a breakup, but like you said, you guys were already having issues. She was looking for an out and that was the last straw.
Most Helpful Guy
That's really immature reason for a break up, son unless she had some problems with that girl and told you that she didn't like her talking to you.
In my case, If she knows I don't like the other guy and still hangs out with him often, then I wouldn't be okay with it cuz that'd be like her betraying me because clearly I wouldn't appreciate that.