Girl narrates how her male friend slapped his girlfriend twice for insulting her

 According to this lady,her male friend offered to drop her off at work but informed her he would be picking up hius own girlfriend on the way.She sat behind to avoid issues and they picked up the girlfriend .The girlfriend started nagging as soon as he wanted to introduce both of them.Then she called her an imbecile,prompting the boyfriend to slap her twice..
Was the girlfriend over reacting considering the guy didn't inform her he was coming with a female friend? Also, for a guy to slap his girlfriend twice over another  lady, friend or not...
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