I do understand your reasons/concerns for not telling the BS.
However, consider this.
My FWH started 24 or so years ago. Who really knows?
I KNOW he had several LTAs (Long term affairs). One lasted for EIGHT YEARS!!!!!
These are the ones I know about. I suspect there were many more.(Read my profile).
At the age of 54 I discovered this.
I had been married for 28 years and KNOW he started cheating when we had been married for only about 4 years.
Discovering what he had been doing devastated me. Totally destroyed me. I will never be the person I was before dday.
During the affairs years I lost both my parents after awful illnesses and found out that both of my children have chronic debilitating life-threatening illnesses.
NOTHING has given me more sleepless nights than my FWH's betrayal.
Everything else I have been able to cope with.
Now I know, FWH is trying to be the model husband.
But he took my life.
He took my chance to have a real marriage. I can't imagine that I will ever be able to forgive or forget.
At my age (57) starting again is an awful thought.
If someone had told me all those years ago. If only......
Maybe he would have woken up. I really believe he would have. Instead he learnt that he could get away with it. Again and again and again.
Maybe I would have left him and found a real husband.
I don't know and I never will.
I wasn't given the chance.
In my opinion, this man will find another willing woman to cheat on his poor wife with.
Do you really want that?
As well, if she knows, you have another set of eyes on them. She will be watching her WH and you two can become allies.
Please tell her. Give her a chance.