I wouldn't tell his wife about it. Although she's a friend, the last thing you need is to be involved with something like that. They'll pull you into the middle of it and they could turn it around onto you and make you the scapegoat for all their problems. I went through something similar with a friend. I caught her boyfriend a few times at a party hooking up with other girls. He also confided in me several time that he didn't love my friend. I told her and, of course, he denied it. They turned it around on to me and claimed that I was making it up because I didn't want to see them happy.
People will do weird things when they don't want to leave someone they thought they trusted.
The best thing to do would be to password protect your computer. Next, restrict porn sites from being accessed from your computer. Also, you're going to want to clear out the crap porn websites puts onto your computer. Porn sites are infamous for putting spyware onto your computer which tracks all your surfing patterns. Delete your cookies and then download AdAware. AdAware will get rid of all the spyware.