I really doubt the husband is ever going to tell her.
I sat here thinking about my half-baked answer. I would want to know, absolutely. Somebody has to tell her. I dare like to bet she knows something is going on already, betrayed people usually do, there are ALWAYS signs of SOMETHING not being right. Just look at how many people come on here asking for advice about why their husband is out late and not calling, always on their phone but deleting the messages or call history.
My ex was up to all sorts, including hard drug use and dealing. He had already slept with prostitutes in the past, and I let it go, but I buried my head in the sand for a while about stuff that was RIGHT in my face. When I asked him about weird stuff I noticed, he called me insecure and crazy, gaslit the hell out of me, then accused ME of cheating.
Cheaters will do ANYTHING to avoid revealing the dishonest person that they really are under the facade. But nobody can keep up a facade forever.
The other thing is that she may well find out from somebody else. She may never find out at all if you don't tell her. But she deserves the truth. And as someone else said earlier, it may well be the one thing that helps her to connect all the dots and stop this b*** from gaslighting her. She could well have serious anxiety from the situation. I know I did. Do you know how horrible it is to just KNOW something is happening in your relationship, but it's constantly denied and YOU are being told you are mentally ill? It is one of the sickest things to do to someone, a real mind F.
Tell her. Calmly, gently tell her. What she does then with her relationship is up to her. And I am sure your son will not be affected. This is nothing to do with a small and innocent child.
Always be prepared for her not to believe you. Someone tried to cheat with me once. I met him in a pub, arranged to meet him a few days later... found out he was married on FB. I messaged his wife and she called me a lying bitch and to stop harassing her family. Be prepared for that too.