Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...

I wish someone had told me about my cheating ex-wife before she infected me with STDs.
One of which is causing me to have cancer decades later.
I understand you don't want to ruin another family, but you are not going to do that, this man has, so place the blame where it belongs. If the shoe were on the other foot, would you want to be told?
Ans yes, I think I would want to know,
Even though I am in the camp that feels only the OM should tell his wife about the affair (since he is the only one who made and broke promises to her), I think the news would come off better from you than from your wife.
Imho the health risk alone is enough of a reason to tell the AP's spouse.