Gosh, I don't think I would want to meet OPs spouse in person. That could be ackward in the future. You may run into them locally... I might consider calling from work and leaving an anonoymous message (or talk to them) and explain something like.... I thought maybe you should know... but it appears that your H may be having an A .... you can include your W's first name or not and any further info you may think is important. But I'd think the less the better since you may not want to become too involved with this. This way you are informing this person about something that is affecting their marriage, but still maintaining a distance.
I'd try to stay out of the drama, and not feed information, or seek too much out at this point. Basically you're just helping eliminate the secrecy with as little gunpowder as possible.
There is no arriving, ever. It is all a continual becoming.