Husband's best friend is having an affair. What do I do?

So I now know that my husband's best friend is having an affair with a woman almost half his age. He's 40 and she's 23 (I previously wrote about this stating she's 21 but I got it wrong and she's actually almost 23).

Anyway I now have proof that she spent the night with him while his family was away but he doesn't know I know. Him and his wife are going through an incredibly difficult period and are currently going through something very difficult to do with one of their children (health issues), this period is expected to go on for at least the next 8 months if not longer. I feel like telling her would not only impact her but would be extremely difficult for the child who is already going through some serious trauma and likely couldn't take on any more. The child has been in therapy but has really struggled to cope, and things are expected to get much much worse medically over the next few months, though she is expected to be (physically) fine in the long run.

What do I do? My thinking was that as long as he doesn't know that I know then we can maintain a friendship but if he knows I know and I'm not telling her then he might take it as a sign I support him and might even try to use me as an alibi. I also think if he knows then it would come out that I knew very quickly once she finds out and that would end our friendship. Right now no one else knows I know.

Regarding STDs, I do want to intervene on the wife's behalf but I don't see how I can do that without telling him to get tested which would show that I know. Any advice?

Edit: I am happy to cut him out and no longer see him however the friendship is long-standing (around 2 decades) and I would like to continue the friendship with his wife and continue to be in the lives of his children. I know he's doing this because he's broken after the recent news about his daughter but I think he is scum now and no longer wish to preserve that friendship. I am aware however that I have to maintain some sort of relationship while him and his wife are together and going through this period.

TL;DR husband's best friend is having an affair, I'm open to telling the wife but feel it's a particularly bad time which could result in piling trauma on to her and their child. But I'm still concerned about STD's and would like to intervene somehow.

Submitted August 20, 2021 at 12:25AM by meksh https://ift.tt/3zc9L3K