Havn't read all the replies but I believe she has the right to know and your friend has a right to tell her, even if it's messy.
IMO I would tell her, but I would to a fair extent expect her to go into denial and either throw it back in my face or claim I'm some weird stalker liar. Hopefully she would take it in though.
Either way I would feel I'd done the right thing and given her the chance to know the truth, as I don't think it's fair to be left in the dark that you are being cheated on . Then I would cut ties and try to repair my own life. Im sure your friend is heartbroken and would need a lot of support right now How awful for her.
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