The other thing is, you have absolutely no idea what this husband is like, what's going on in that marriage, anything.
Supposing he's a deeply scary person - worst case scenario , he becomes obsessed with you? It can happen.
I'm guessing that you have a story in your head about how you'd like it to go: you send the letter, he's furious, kicks her out, and she is left in the position you're in now. Which, I suppose, would lend a certain (illusory) satisfaction. Or he'll batter your ex to a pulp. (Which I, personally, find quite unappealing, but who am I to judge?)
Who says it'll go that way, just because that's what you'd like.
No, the more I think about this, the more I think staying out of it, concentrating on you, and your life now, is the way forwards. The best revenge is living well.