Okay really complicated FWB's question (friends with benefits)?

Okay so me and my best friend both told each other we like each other but at the moment he has a friend with benefits. And it's standard "rules" for FWB's. Do whatever you want, not be attached to one another and break it off if you wanna go out with someone unless it doesn't last then they go back to each other and no secrets. This really irritates me because I feel like I'm gonna die everytime he tells me he's done something with that stuck up snooty little know it all of a c*n*. I think me and him actually really like each other because when I told him this we watched a movie after then I put my arm around him and he snuggled right in close to me like his head on my shoulder and his hand on my leg. He knows how I feel about her but he said it's just so she doesn't kill herself because they already had and she gave him her virginity and if he cuts her out of his life completely she'll kill herself.Has anyone been in this kind of situation where your gay/ bi, he's bi but FWB's with an insane psycho stalker chick? Girls your free to answer as well just reword that from he to her etc. And the virginity thing I don't know how that'll work out but let's go for it. And do you think i should possible ask hin out? Please I need some answers here :) serious ones