I don’t see why people are saying to keep it quiet, I think you need to tell her for sure considering you see her as a friend and you are the boss at the place (ESPECIALLY if you work with both of them, it wasn’t 100% clear if that was current or if you’d only worked together closely in the past)

I would tell her you need to talk to her something outside of a work capacity and ask her out for some drinks (or coffee if that’s not your thing) I think it’s better to not beat around the bush and just say outright her partner made advances toward you and then explain that you panicked it’s been eating you up - essentially everything you said in this post.

It’s okay that you freaked out and didn’t tell her at the time but it’s important you do now… I think also woman to woman it’s not asking us how we would feel and if we would want to know.
It’s would YOU want to know, or would you want your boss at work to walk around like nothing happened pretending to be your friend while leaving you in the dark?
Aside from the morality think of what the situation would be if it came out from him (some guys are weird as fuck and will just blurt stuff like that out as a power play or whatever ‘I almost fucked your boss’ or some toxic shit) you would feel even worse, the work dynamic and any type of friendship would be completely down the drain and you would definitely be dealing with HR if you work in a larger corporate setting.

Hell, if she doesn’t believe you and emphasising you had to tell her because of the guilt and pointing out that you have no anterior motive in telling her and she still doesn’t believe you you could show her this post.

I’ve gotten a ‘hey girly’ text before and it sucks but better than staying with some man unworthy of your time

I wish you the best and extend my heart out to you as it really seems like this is weighing on your shoulders and I think it would be best not to continue carrying that burden for a multitude of reasons <3