love triangle

So my friend has this crush on a boy and she has liked him for 2 1/2 years. Any ways, he has been flirting with me for the past couple months. It has been irritating but satisfying. Now I actually like him and have feelings fro him. I feel really ba fro my friend but I want to ask him out. Idk if i should tell her what is going on or not. Please HELP!!!!