I (35f) was in a casual sexual relationship with Tom (50m) over the span of years. All I asked of him was to be honest with me about when he was having sex with someone else, both for my health and honestly just out of basic respect.

After I saw photos of him in a suggestive pose with a woman on social media, I asked him whether he was having sex with anyone else - and he admitted that he had been having sex with multiple women (including the one in the photo) while having sex with me.

His intentional duplicitousness was a dealbreaker for me, so we stopped the sexual relationship. I wanted to let the other women know (again, for the sake of health and out of respect) but I had no way of knowing who they were in order to reach out to them.

Fast forward to now, and I know who one of the women is. She's (27f) still in a relationship with him. It's eating me alive to think that she believes their relationship has been honest the entire time. (There's no way he's told her about how their relationship started. It sounds presumptuous but I know this man well enough to know that much.)

I have the means to tell her but should I?

Why I might be TA: All I'm going to accomplish is sewing discord. All that's going to happen is sadness or anger or disappointment, and I honestly want happiness for both of them. Is it even my place to tell her?