Somewhat overly simplistic B/G: I caught a friend and colleague of several years, let's call her Nellie Oleson, in a slanderous lie about a colleague. The catch was completely accidental, but the lie was intentional and something of a whopper.
The friend who accidentally facilitated the catch in a casual conversation was likewise surprised by this development. (I didn't know at all that I had been lied to, so I kept trying to clarify what had happened and asked for more details, which led to us figuring out at the same moment that this was a fish story.) Let's call this friend Mary Ingalls. Mary Ingalls and Nellie Oleson are still friends, but I have done the drift and disappearing act on Nellie. Part of the reason I have kept my distance is that I am angry, and part because I don't wish to continue the relationship, but also don't want a big confrontation. Mary and I have agreed that should Nellie ever ask for an explanation, I will be truthful with her, but none will be offered if she doesn't ask. (Mary feels a little bad that she unintentionally helped the lie come to light.) I am quite good at dodging conversations I don't want to have, but I'm not sure that's the right thing here.
So, Nellie and Mary and I will be at a function together. We all work in the same industry, which is one reason I don't want a giant confrontation. In the event that there is an inquiry (she's asked another friend if she'd "done something" to upset me) - I would like a gentle, polite way of saying I know about the lie. I don't think I need to say this is why I've been scarce, because I think that's self-explanatory. I just want to convey that I know and she can think it over and do what she wants with that information (hopefully learn and not repeat behavior).
Is there a way to do this briefly, gently, and not be ugly? For one thing, if I make a vague reference to the incident, she is quite likely to feign forgetfulness - it was 2 months ago now. I don't want to lie if asked directly, and I also don't want to blow her hair back, so I'm trying to prepare myself. Any ideas? Or should I handle it in an all together different way?