I think my wife is in love with college kid half her age

My wife is 45 years old and we've been married for 8 years and together for 15 and these years have been truly great in all possible ways. We still travel together often and have a great time. I cook food for her every day and she still gets breakfast in bed every weekend. She talks to me about everything and we basically have this perfect marriage. Or so I thought. A year ago she went back to school because she always wanted a litterature degree. Back in April she started acting a little weird in regards to one of the 22 year old male students in her class. It got to the point that I asked her if something was going on with this guy, but she assured me it was only because he had been nice to her and she felt lonely and she didn't want to lose his friendship. He kept coming up and we had 3 serious talks where she assured me that everything was completely blown out of proportion. I accepted that and moved on. Summer passed and school started for her again and I figured everything would be ok and it was until her computer broke. Since I'm an IT consultant she asked me for help and I, of course, did. That's, when recovering her files, I came across a hidden diary she apparently has been coming back to everyday and entering everything about him (clothes he wears, music he listens to etc etc). I didn't tell her what I had found and just returned the computer, however, and I HATE myself for this, I have tracked the use of the specific folder and she keeps going back to it all the time entering things from his Facebook, listening to his music. She doesn't write any personal feelings or anything, but every night before going to bed she goes back and read about him. I might also add that he doesn't write her and to my knowledge he seems completely unaware of my wife's doings. I brought him up again the other day asking if she is still thinking about him, but she told me no and that the whole thing was blown out of proportion before. Well, her behaviour says otherwise and as much as it breaks my heart I can't see any other explanation than that she's in love with him. 150 students and not a single mention. Only him. Every day. To put it mildly, I'm a broken man. I love my wife dearly. I'm 99% sure she hasn't had any physical contact with, but it doesn't help because emotionally she seems to be with him all the time and it tears me apart. I'm virtually dying from sorrow. What should I do?