Should I tell my friend I saw her boyfriend cheating?

Ok, i was in a very similar situation, except it was my ex bf that a ratted out to his gf. For 2 1/2 yrs i tried and tried to tell her hes being unfaithful to her, and he constantly lies to her. She ALWAYS ended up believing him, because we never liked each other. 2 weeks ago, we became friends, and she now believes me. He is really upset that we are friends, but thats his problem. She has been seeing someone (hes been using some girl again) and told her that i told him she was seeing “tyler”. She totally believed him. I couldnt believe what i was hearing. Like i would become friends with her only to stab her in the back?..is she kidding? I was so mad i texted him telling him that i dont appreciate him accusing me of something i didnt do. I told him what she does is NONE of his business, and that he needs to stop being the coward he is by lying all the time!

Some people would say that its NOT your place to tattle, but if my bf cheated on me, i would appreciate someone telling me. I think you owe it to your friend to tell her, but then again it doesnt sound like she is capable of accepting the truth. She sounds overly obsessive, which isnt good at all! So id have to say..ITS YOUR CALL!