Relationship help (another girl)?

My boyfriend and i have been together for almost 10months, and lately we have been arguing about one girl. Her name is Mjay and she is the ex girlfriend of a mate of my boyfriends. She has been flirting with him all the time and she is always next to him rubbing up against him and always looking at him and they are always talking on facebook. This girl is annoying the Absolute crap out of me and i am afraid of losing my boyfriend. What should i do?