I have My Best friend who dating her boyfriend for 8 mouths and at that hole 8 mouths he been taking people that there not together and thing... but she been se...
But The Thing she knows because I told her but her boyfrend doesn't know she knows everything... so that why she been asking me What the best revenge to give a guy?
Tell her the truth. She is gonna be more upset that you, her best friend, didn't tell her. If she hears it from someone else she wont be able to trust you.