2 weeks of wham bam thank u ma'am VS 6 months of solid friendship...

Sorry but in this particular situation I would choose to forgive my friend over a booty call.

He was never yours to steal anyway, and truth be told, he chose her freely. If he was thinking with the head you're assuming he is, then he would've either stayed with you, or tried having both of you as fwb. He wouldn't be relationshipping. If I would be mad at anyone it would be him, because I was lead to believe that he only ever wanted casual sex because he's not the relationship type, and then BOOM! He dumps me to be my best friends bf... Like wtf, what a total liar and a douche.

Now my friend, if she knew I liked him as more than a booty call, then yes, shame on her and it would hurt like hell. But if I never told her I was into him... Well, people aren't mind readers.

Really sorry you're hurting, I was in a similar situation at one point, lucky for me I chose to forgive my friend, because a decade later the boy is long forgotten and I still have my friendship in tact.

I hope you start healing from this experience soon, and also give the opposite gender some credit, they do have brains and can use them when they choose to.