Did you ask this person outright if they were in a relationship?
Was it lied about or just never brought up?

You said it was by pure accident you discovered this, what do you think would’ve happened if it continued?

(I’m not pointing any fingers, I’m just wondering what the other side of this looks like if it’s possible)
I’m sorry this happened to you & im sure you feel outraged by it.
But does your outrage of being duped justify telling the other women.
It’s just not a very black & white sided situation.