I'm going to call my friend a bitch
I'm really not understanding why you'd call your friend a bitch?
I'm just returning the "favor" to her.
Don't stoop to her level by name-calling. Talk to her about calling you names. If she continues do it, then end things with her.
Not everyone can be nice all the time.
Just because you can't be nice all the time doesn't mean that you shouldn't strive to at least try. Not calling your friend a bitch would cost you nothing, and is the right and nice thing to do.