You could just send her a polite message saying you know what happened and you no longer consider her a friend, and that you would like her to stay well away from you at any social occasions. Or wait until you see her and say it face to face.
If you don’t say anything then she will think that he hasn’t told you, and might even build that up into something in her mind.
Years ago, and old friend of mine tried it on with my DH, shortly after I’d started seeing him. He didn’t actually tell me until a couple of months afterwards though as he didn’t want to create drama.
I hated the fact that I’d seen her in that time though and that she knew that he hadn’t told me, and it felt as though they had been keeping a secret from me in a way.
I didn’t fall out with her but the next time I saw her, when we had a minute on our own, I told her that he’d told me what happened, and I said it quite calmly and nicely.
She was very embarrassed and apologised and said that she was very drunk. I left it there and didn’t bring it up again. I knew that he wasn’t interested and that he never would be, so that was that. We aren’t close friends anymore though (and we were before, since childhood !). The friendship has cooled off for many reasons; I just couldn’t see her in the same way anymore. I am polite when we do see each other though.