Kelly this is gonna sound really harsh but ya i tell it straight up.
Theres a good reason the two of you broke up, so maybe you shouldn't be thinking so much what hes doing. As for your "friend" sounds to me like she's a slut, dunno why you can keep calling her a friend if shes a slut. Maybe you like having slutty friends? If one of my friends starting sleeping with an ex and hadn't asked me if it was cool, they would no longer be a friend. Cause FRIENDS dont do that to eachother.
Now if a friend asked me if it was cool to see an ex of mine and I said "ya its cool" even though it didn't feel right well then thats something I have to deal with.
I think you need to simply move on, feel bad well go spend some quality
time with the current bf. There are sooo many messed up and immoral people in this world. If you call one of them your friend thats up to you. Though I think you need to be thinking about your "friendship" and not so much about what they're doing, actions reflect the type of person the two of them really are; choice is yours.
I'm gonna add, girls (including every last one of you) want what you can't have. Get over yourselves...