what happened to the guy she cheated with? why didn't she just go with him? or was she masturbating?

This is not the sort of person to be around. Tell someone else who cares about her she said this, and get out.

Run.

Let her.

To people saying 'let her', I think that is not okay. This woman is clearly in the wrong, and I am guessing most people here think it is probably a bluff, but we don't know that. She is a human being who has cheated and may be being manipulative and/or genuinely feeling suicidal. But do you really think she deserves to die? She may well be bluffing but if she isn't, then she clearly has mental health issues and needs professional help. OP, please get out of this relationship - she has no right to do that to you, but please inform someone who can help her, like a family member.

pretend to forgive her, then leave.

She is a drama queen. I hate to say this, but let her. She obviously doesn't care about you. Release her and don't forgive her. I don't want anybody to kill themselves but if they threaten to if they are not forgiven for cheating on someone, they are bluffing.

Call her bluff. Wither she does or not, at least she'll be out of your life.

Let her, natural selection at work.

Yeah, tell her to go F herself and enjoy the grave then.