This is OP. Thanks to those who have offered their perspective, different as they may be. Additional info a few PPs requested:

1) DH claims he’s only known for a few months, not 8 years.
2) Best Friend claims he and his DW are sexless so there is no risk of STD transmission.
3) Their DC is 10. Ours are a bit younger.

My brain tells me I should mind my own business, particularly if point 2 is true (and Best Friend has no reason to outright lie to DH), but I still feel sick.

Anonymous wrote:

Anonymous wrote:I would be more concerned that my husband seemed to think it was ok to keep this secret from you for 8 years while your families kept bonding and getting closer. I would find this really upsetting and question my husband’s character. My husband has had a few long-standing friends of decades make questionable life questions. It results in the friendship cooling off considerably.

+1. Guys covering up for other guys makes me sick. Low morality all around. Stupid bro code excuses inexcusable behavior.

+2, I find this so gross, unless this is actually cool with the wife.

Anonymous wrote:Op, when the affair is discovered and your friend finds out that you knew and don’t tell her, you will loose that friendship. Speaking from experience here.

She is going to lose the friendship anyway when the couple divorces. The husbands are best friends from college. Why would either he or OP stay in touch w best friend’s ex wife?

OP, is this woman your friend or is she your husband's friend’s wife? There is a difference. My husband has plenty of friends whose wives I see socially every now and then but if DH and I were to divorce I imagine I would never see again.

Play this through to the end - how do you see this working out if you either tell her, or don’t tell her? What do you envision your role to be in this, and why? Just because you know this info you feel you must share it? Or is there another reason?

Anonymous wrote:

Anonymous wrote:I guarantee you she already knows.

Not necessarily. My uncle had a decade long affair and when my aunt finally found out, she divorced him immediately.

This happened in my family. The betrayed spouse not only divorced but immediately cut off all the in-laws who had not only known but also abetted the affair. The kids were teens and stopped speaking to them too.

Maybe the friend and his wife have an agreement.

Op, when the affair is discovered and your friend finds out that you knew and don’t tell her, you will loose that friendship. Speaking from experience here.

Anonymous wrote:I guarantee you she already knows.

Not necessarily. My uncle had a decade long affair and when my aunt finally found out, she divorced him immediately.

Anonymous wrote:I (and most wives) would want to know.

I would discreetly talk to her and say,
"Mary, I can't verify, but I learned that your DH may be having a long-running affair with X. I hope it's not true, but I would want to know this if it were me. I won't mention it to anyone else, and you can explore/investigate it or not. I just want to let you know I'm here for you and will help in any way you need."

I would not. I don't think you can speak for most wives.
OP's DH needs to demand his friend end it.

I agree it's not your place to say anything. But you do need to tell your DH that he has to tell his friend to end it.

Anonymous wrote:I would tell the wife a million times over, not even a debate in my mind. A friend of DH's finally caved and told me and I will forever be thankful for him. The one person that actually considered me, my health, my agency, and the fact that I was making major life decisions under the assumption that I was in a good, two-person, monogamous marriage.

Unless your partner has had a long term affair that blindsided you, you don't really have the experience to weigh in here.

+1 from someone who was also blindsided

and don't tell your DH that you know,
but maybe start checking his phone and GPS. never know what he's up to.

My DH started his affair after learning a friend of his had one. It's like it gave him justification

I (and most wives) would want to know.

I would discreetly talk to her and say,
"Mary, I can't verify, but I learned that your DH may be having a long-running affair with X. I hope it's not true, but I would want to know this if it were me. I won't mention it to anyone else, and you can explore/investigate it or not. I just want to let you know I'm here for you and will help in any way you need."

I would tell the wife a million times over, not even a debate in my mind. A friend of DH's finally caved and told me and I will forever be thankful for him. The one person that actually considered me, my health, my agency, and the fact that I was making major life decisions under the assumption that I was in a good, two-person, monogamous marriage.

Unless your partner has had a long term affair that blindsided you, you don't really have the experience to weigh in here.

Anonymous wrote:I would be more concerned that my husband seemed to think it was ok to keep this secret from you for 8 years while your families kept bonding and getting closer. I would find this really upsetting and question my husband’s character. My husband has had a few long-standing friends of decades make questionable life questions. It results in the friendship cooling off considerably.

+1. Guys covering up for other guys makes me sick. Low morality all around. Stupid bro code excuses inexcusable behavior.