From a woman's perspective. .

I think I'd let it go too. . . and of course you will get a why.

For some reason this woman chose not to involve this man with this boy -- maybe not even financially (which is odd for this day and age). While there were plans to do so, there may not have been plans for this to be a "family unit". He may have simply been a "natural donor to the cause". (Not that THAT has EVER happened!)

So let's say you go to your friend's parents and tell them of this conversation. Their son had a relationship with this woman and there is a child. This child may in fact be the only living legacy left of their son. But you don't know this woman, you may not know her name or where she is. They might know her or not. If we don't know this person, how will they ever go about finding her. Then they will sit there and stew about this long-lost grandchild.

or

Let things take their course and someday maybe the mother or child will find them. . . and they will have a wonderful surprise actually in front of them that is their son's living legacy that they can hug and hold.

And one more thought. . . . there might be one other reason this man wasn't introduced to the child earier or asked for support or whatever. A DNA test might prove otherwise. (Not that THAT has EVER happened) I would hate to put his family through all that turmoil. I can't imagine the family scouring the countryside for the long-lost grandchild only to find out later that it truly wasn't their sons. Toooo crushing.