That shouldn't be boy trouble, that's an easy one, leave him alone! If he's flirting with you tell him to stop as he already has a girlfriend. If he breaks up with her, then that's a different story, but right now, leave it be.
I'm having boy trouble. My friend is going out with a new found crush of mine, he's very flirty with me, but is going out with my best mate. What do I do?
Not sure what 'trouble' you are referring to .. The fact that you best mate is going out with him, which is defined as a 'relationship' .. Or that he is flirty with you even though he is supposed to be in a monogomous relationship with your friend.
Tell your friend that he is flirting with you.