Do I tell his partner?

[Conversation16]

I have recently come out of a 9 month long affair with a man who was living with another woman. It ended with her telephoning me, telling me that she knew I was phoning him, that they were planning to get married and I should leave him alone.
In fact, we had been talking daily, and had met up, and slept together, several times.
On the phone I acknowledged that I had been phoning him, but nothing more. The relationship with him has ended.
I am now (and I realise that it's 9 months too late) wondering what the 'right' or 'best' thing to do is in relation to telling his partner. Is it helpful/ hurtful for her to know that he had been sleeping with me, and intensely involved with me, in light of their marriage plans (which I knew nothing about)? Is the least harm I can do just to back right away?
Not expecting any understanding for the relationship, and prepared (I think) for anything thrown at me, but also genuinely want to know if people think I should tell this women an important part of her partners very recent past.

Conversation16 How very noble of you.

You didn't care about her when you were sleeping with her partner, why pretend you care now?

She probably already knows he's a lying cheating arsehole.

Leave her alone and sort your own self out.

joysmum - I was cheated on and that is what informed my earlier reply which others seemed to object to. I wanted to know. I wished someone had told me earlier WHILE the affair was ongoing.

I found out just after the affair had 'ended' (ie OW told H she couldn't handle getting so little of him so they would just be 'friends' ). Once I found out it ended for good. He ended it, permanently.

What mattered to me at that point was that

1. He got too close to another woman - whether he was shagging her in the stationery cupboard or simply writing haiku in praise of her eyes made no difference. He had done something utterly hurtful and selfish.
2. It was over for good.
3. H told me what had happened in detail.

3, I had a chance to move forward in whatever direction made sense to me.

1. Every and anyone else should butt out unless they had evidence that the affair wasn't over - that I would want to know!

I couldn't give a stuff about what the OW thought I should know. Her honesty was worth less than nothing. There had been a time when it would have been meaningful but that was before dday. Whatever she had to tell was bound to be coloured by either resentment, the desire to hurt me and H, or fear (that I would tell her H).