For some reason, some 20 something girls seem to go for older dudes. I've never understood it, nor have I ever done it. But it's possible. DO NOT discount it just cuz of his age.
Not that I mean to thread jack but in answer to your statement about not understanding why the twenty something ladies seem more interested in older men, you are right but there is a reason.
Forgive my stereo typing but a good portion of the twenty something males never grew up. They are still sitting in mom and dads basement playing video games and getting stoned. Not a lot of motivation or drive. So when the twenty something women are looking for a man they look at the men their age as boys. Then they look at the forty something guys out there that don't play video games, have a good career, do manly things like fix their own car and make repairs around the house and they quickly see that the older guys maybe don't have the astonishing good looks of youth but they are very masculine manly men, not little boys that need a mommy.
As we've always said here, women aren't attracted to weak men. A lot of the Metro sexual video game junkie guys today come of as pretty weak and immature, neither of these are attractive qualities.
as far as the original post goes, I'm not sure why but I remember reading something almost identical about a year ago. Not sure if it was here or on SI but it is almost the exact same story just a couple details with some minor changes. Maybe coincidence or maybe just some new fishing, who knows.
In any event assuming its a legitimate post I will answer like this, It sounds to me like he is friend zoned. She needs somebody to get stoned with and he is better than nobody, he's conveniently located right next door which makes life easier to get a buzz with little effort.
If everything you have said is accurate than I don't think there is much to worry about, but, and this is a big but, This is only if you are being honest with yourself about your evaluation of him.
Have you decided he is a loser and as such you don't see any of his good qualities or is he really the mentally deficient loser you have made him out to be. is he physically attractive/ok? IS he able to communicate with your wife on a level that is consistent of your wife's mental acuity?
Don't look at him and evaluate whether or not you like him, look at him and evaluate him from the perspective of whether or not he has anything to offer a potential partner/mate. If he is as bad as you say than I think the relationship is very likely exactly what she is telling you, not that it can't change in the future but for now I think it is what she says it is.