Is it ever okay to stay silent if you know someone is being cheated on?

Throwaway account.

I recently found out that someone in my social circle has been cheating on his wife for almost a year.

This isn't a rumor. My husband found out because the husband admitted it.

The wife doesn't know, and she isn't one of my closest friends, but we've known each other for about four years and see each other around once a month. We even spent several days camping together recently, and I only found out the truth after we came back because my husband didn't want to ruin the trip for me.

What makes this especially difficult is that, shortly before the trip, she and I had a conversation about cheating. She told me she couldn't imagine cheating on someone because she wouldn't even be able to look them in the eye afterward. She also said she thought she might forgive her husband if he cheated.

Now I know he has been cheating on her for months, and apparently has no intention of stopping. He has even admitted that, ideally, he wants both a wife and a mistress.

I don't feel like I'm the right person to tell her because we aren't close enough for me to suddenly change her life forever. At the same time, I don't know how I'm supposed to keep seeing her and pretending everything is normal.

So my question is more general than just my situation:

Is it ever okay to stay silent when you know someone is being cheated on, especially if you're not one of their closest friends? If you've been in this situation, what did you do, and do you regret it?

Edit: This post was written with AI because English isn't my first language, but the situation is real.