How do I tell this girl to back the F off??

Anonymous
My boyfriend got a text message from a girl saying that she has a serious crush on him and she wants him to break up with me and she wants to move in with him. He told her he \has his good days with me and his bad days. He showed me her messages. Well this morning while he was sleeping I looked at his phone log and saw he called her twice the day before! They talked for a long time! This morning when he woke up she text him "Good morning :)"

What should I do? Should I give my boyfriend and ultimatum? I am seriously getting p*ssed over this. Should I tell him to let me talk to her or something?
How do I tell this girl to back the F off??
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