One of my friends likes the same guy as me..?

[amm]

My best friend is a guy, and I have a friend (who is a girl) who likes him. Turns out, I've dated him before, and so has she. I like him still, but I don't want to ruin any friend relationships with her over him. I really want him to ask me out, but I'm afraid she'd get mad. We get in fights a lot about him, and she's really mean through text or twitter. But the next day she sees me in person and is my best friend. What can I do to explain all of this to him, and tell her how I feel without starting a fight? I love her to death as a friend, but sometimes she just drives me crazy!