grumblleweeds
Don't tell the wife.
You can't possibly know for sure from such a hearsay account that it's actually him.
It will cause a bomb to go off in your friend's life and if he denies it she has zero proof do she's unlikely to end it over gossip.
Plus, and this may be unpopular, people have affairs all the time! Most times they fizzle out. I would actually go so far to say that in a long marriage (30-40 plus years) the chance of one of the spouses playing away at some point is probably very high. It doesn't mean they don't have a good solid marriage.
Don't interfere would be my advice.
It's not a good solid marriage if you're shagging someone else. Stop making excuses.
Don't be so quick to judge. Life is long and all marriages have their ups and downs. Of course, it's a shitty thing to do but people are not perfect. Ideally, it would never happen but we don't live in an ideal world. I'm not making excuses or condoning it but it happens. A freaking lot. I'm old now and I don't know any marriages that have not been rocked along the way. Particularly at the stage of the marriage where there are young kids. Again, I am NOT condoning it before I get piled on.
That's why I say keep out of someone else's marriage.