Also, who says maturity has anything to do with it or it is he who is not mature? This girl could be the one being immature. In this culture, people tend to think that the guy is always the suspect and is at fault and the girl isn't, but that is far from the truth. I don't like it when people think that.
Based on his post, this is what happened:
He and his girlfriend went out. She met somebody she knew from school and talked with him for a long while, ignoring peterv.
Peterv, instead of attempting to join the conversation or attempting to draw her attention back to him, instead sulked for a hours and then left without telling her.
THEN, he decided to pout some more and ignore her phone calls.
After that, he decided to break up with her.
Do you see it yet? He gave limited information, and I'm well aware that that is the case, but it is safer to judge only on what he actually said rather than what maybe could possibly be the case. Based on just his post, the girl was being insensitive and rude. Insensitivity and rudeness are things you talk through. You say "hey, I felt pretty ignored back there, next time could you introduce me and try to invite me into the conversation?" The only reason that would be difficult or impossible is immaturity.