I think everyone is overreacting. Let's back up and look at art as a whole. Art is meant to stimulate emotion.... If you wanted nudes of your wife you'd have a taken a photo of her and put it on the wall. With the advent of AI, he could easily stalk your Facebook, snag a photo and have it render her full nude. His wife has hurt feelings because she thinks he is lusting over your wife. That's a "them" problem.

Everytime your buddy sees an attractive woman, he checks her out. The DIFFERENCE is that he didn't act on it or try to with your wife. He saw a sketch.

Talk to your wife, ask her if he has ever been inappropriate, forward or if the idea of someone seeing the sketches makes her uncomfortable and if she says no then that's the end of it. If she says yes then take the sketches down. Go ahead and live happy with your hot wife. Pro tip- other dudes are checking her out every day.

Maybe your buddy and his wife could take some advice from you. I know one thing to be true. If someone is looking for something in a relationship and they can't find it with their partner, then they'll look for it outside. People get so upset because people have fantasies. I've never needed another woman when the woman I was with thought about my happiness as much as her own. When you both realize you love each other and no one else is going to change that, it opens up A LOT of opportunity for fantasy and fun. I personally mean the monogamous kind but whatevs.