Are you sure it's sexual attraction? She could just think of him as a son. I've gotten close to my kid's friends but I tend to think of them all as my babies. I do think visiting him at work and texting him all the time is a little strange. My question to you is why do you feel threatened by a 20 yr old kid? Your wife's a little crazy, but it's probably just a phase. Tell her to pull her head out of her butt or she'll be looking for another place to live.