Dilemma - Best Friend is Cheating

So one of my best friends got married in August. Our husbands are best friends, my husband was the best man in their wedding. I recently found out that she has been cheating on her husband pretty much since right after the wedding.

She’s been avoiding me and my husband for the most part, I’m assuming in fear that my husband will somehow catch on that something shady is happening and tell her husband.

I tell my husband everything, and it’s so hard to keep this from him. On top of that, I feel like a liar every time the 4 of us are out together I can barely even look at her husband anymore.

Do I tell my husband? Do I confront my friend? Do I keep my mouth shut?

I’d want to know if I was being cheated on. Idk what to do.

EDIT - To answer one of the questions below: I know because two of our mutual friends that she confided in assumed that she confided in me as well and let it slip out in conversation. My husband does not know the “other man”. Her husband has met him in passing - it’s someone she used to work with.