I just thought of this, but I would not at all bring up the shower incident. Because you didn't say anything at the time it happened, to him but especially to her, it will look like it didn't bother you all that much. That, coupled with the letter, could make it seem like you encouraged this or at the very least didn't discourage it. Focus on the here and now: the letter.
And I just noticed this post of yours where you encourage the OP to lie. There's goes her moral authority and her friendship. On one hand you say you can't live knowing someone is "living a lie" but you recommend they tell one?
This would make the OP not just a meddler, but a player. The last thing Lisa needs is more lies. The kindest thing that the OP can do for herself and for Lisa is to just distance herself. It doesn't involve any lies. And if Lisa contacts her and wants to talk about it, there is no need to lie--the OP has made her position clear by staying completely out of it.
I never said lie, I did say to omit an incident, but in order to not make things worse for her (the OP) than they already will be. If it were a closer friendship, sure, I'd encourage her to tell Lisa and say, "It was odd then and I should have said something, but I didn't and I'm sorry." The fact that losing this friendship doesn't seem to bother the OP makes me think it is best not to tell Lisa. The time to tell her would have been when it happened, but there is no changing that now.
I don't think the OP telling Lisa about the letter is meddling and I don't understand how you do. Could you please clarify how that is "meddling"? I also don't understand how you think distancing herself is not lying but not telling her about the shower incident is? Those seem to be the same to me: omitting the true events.
The reason I thought the "life is not a movie" comment was a dig is because it seems like you think I think it is and that you think I think it will play out like a movie where the bad guy gets the comeuppance. I don't see why else you would have included the comment without more explanation.
Let me address the movie thing first. When you don't have first hand experience with this sort of thing you can have a sanitized view of how things will play out. It's not just you. I am continue to be amazed at how things play out much differently than I expected them to.
OK, onto the meddling. The OP is not a principal actor in this drama. She is at best a walk on role in terms of these people's marriage. It's just an unfortunate position for her to be in. And the best and most decisive action she can take is to get out of that role and not play any other role in the drama, like that of informant or confidante or supportive friend.
Emotions are likely to be high and unpredictable. Even if the OP can prove that she did nothing to provoke his interest, that doesn't mean that Lisa will see it that way.
Yes, it is possible that she would be very happy to know about the letter and will act on it, etc, etc and even thank that OP. But, the problem is that you can't really predict which one its going to be.
I have learned the hard way a few times that, if at all possible, its best to stay out of other people's business completely when its something messy like infidelity.
Everyone in those situations gets burned in some way or another. The only way that I would consider telling someone something I knew they didn't want to know about their spouse is if they were a very very close friend. And even then, I would try to avoid it, if possible.
You might think you are "saving someone" but a lot of the time reality plays out much differently.