Did he cheat? Was it just friends?

I have a friend whos boyfriend went on facebook and flirted with another girl. She didnt know her boyfriend even had facebook. Anyway she seen messages in his facebook one day and in his inbox it had multiple conversations of him and another girl my friend didnt even know. he was saying to the other girl ; hey hun i missed you xxoxoxo, we are getting to know each other better and thats a good thing, oh sorry hun i got caught up last night but i did wait for you i missed you xoxoxoxo, we should meet up and hang out xoxo, your kinda cute ;) ..aww so are you, here i want you to have my number.. Anyway these are the things he said to her and she said to him. He didnt tell his girlfriend that he was talking to her. He claimed she and him never talk as much and shes not their for him and she drove him to talk to someone else. Should my friend be upset or forgive him? And was it cheating to you?