There's only one thing you can do! Get her to dump the ragequitting BF get with the calm guy. Basically NTR him. Only solution. On a serious note. Just don't play competitive games together. FYI he only asked his GF to meet you so he can compare himself with you. He's looking for threats. Many possessive people tend to do this. Then again what do I know I'm just a rando on the interwebz.
hmm... It's hard to instantly change the mentality in stormy situation, it keep linger and stick around although you tried to calm your self.. I tried counting numbers and it didn't really help..
the thing is I dunno why but she keep getting head strong man for bf I know her since junior high and her bf so far is really similiar with each other..
Most of the time your face get contorted because you are still thinking about the events that upset you. Go to the restroom, wash your face a little, and snap out of it. It's just a small matter that won't really affect you in long term, so learn to let go of your upset feeling. After you get back, try to laugh once or twice at your friend's joke/try to make one yyoursel.
Tho, IDK what means to stay calm is, you don't do anything wrong... perhaps, but If I'm in the circumstances where I can't calm like after making pranks, to make myself calm = if a guy I hug him (and start discussing something, I means hug makes your calm?) and If a girl I use phone (just doing whatever to stay out of nerves and focus on phone; ultimate avoidances)
Buy him a drink. He'd calm down after a drink. Then just make light of the whole situation or bash yourself over it in a joking tone. Also you're a girl ? Or not. It's strange if someone would get mad and curses to a girl over a game.