[gars]

I wouldn't tell the wife. While you might think that it's the respectable thing to do - I think I know some women would rather be left in the dark. What if he decides that the 20 yr old isn't good for him, and goes back to his wife - but because you told her the truth, YOU would be the one she would hold responsible for ripping apart the marriage. I would also refrain from being the guy who tells him to divorce his wife. Again - the 20 yr old might be the one who is the true homewrecker, but you would still be the one who ultimately breaks them apart.

Honestly, I highly doubt that the 20 yr old not know that he's married - unless he's been keeping it from her - leading a double life. Either way, their relationship is built on cheating or lies - not a good foundation for a healthy relationship that can/will hold together for a longer time. Hopefully he wakes up and smartens up and just leaves her - maybe even suggesting a marriage counselor, which might be enough to revitalize the family.