What would you do?

Okay let's talk about friends cheating on you or girlfriends being interested also in every guy you are with. If this situation comes what would you do? How would you react and how will you deal with that friend?

The idea came when my bestfriend Emy who is currently in the UK posted a bulletin regarding this situation. That's totally annoying and I would really be pissed off if that happens to me. So here goes her bulletin:

Ok, i have a big problem with my english friend! Recently, ive started dating this guy that i REALLY like and he really likes me.Both in love now actually.
But today, my boyfriend came to meet me like he usually does, and we were in the middle of a conversation and my friend just came over and clinged on to him, bare in mind now we were like holding hands.
And i just sort of laughed it off, but then she was like, will you come with me to find my coat? (To My boyfriend) And he was like i want to spend time with (My Name). Soo Then she started like leaning on him and stroking his arm and that!
I was so shocked! .. because she's like flat out flirting, right in front of me!
Now this is crossing the line, so what can i do to tell her to back off?
Thanks For Your anwsers.
Much Love x
em

The nerve of that friend. Oh well, we have different personalities so I cannot point out what she is up to. I gave my friend this advice.

TELL HER TO BACK OFF AND GO TO HELL. SHE HAS NO RESPECT OF YOU BY DOING THAT. YOU SHOULD NOT COUNT HER AS FRIEND. ALRIGHT SHE IS ENGLISH AND LIBERATED MAY BE, BUT STILL, IF SHE IS A FRIEND SHE WOULD NOT FLIRT ON YOUR BOYFIE STRAIGHT TO YOUR FACE. SHOW her WHAT YOU'VE GOT. DON'T LET her STOOP YOU DOWN. SHOW HER THAT MARK IS SEALED AS " OWNED BY EMELOU ALONE" HAHA LOL..

How about you. I trust many of you are expert on handling cheating situations or that kind. Would you give me a piece of your thought? Thanks.