what would you do if the guy your seeing kisses another girl?

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my friend is seeing this guy and he went to the footy and this girl was there (mind you we had heard rumours that they have previously talked when she's been seeing him and he claimed they didn't), he had left my friend on read while we was there and my friend asked him if anything happened between them at all and he denied and said he would never go there with her or dog her like that, we then heard and got proof that he kissed this girl and denied it up until the point where she sent him proof of the girl admitting it. She doesn't know what to do as he claims "he's so stupid and he loves her and only her and regrets everything he did as he was drunk", she still has feelings for him but doesn't want to get hurt if she continues seeing him.