I would have a serious conversation with him, now. I am sure classmates are already talking, and he needs to realize this is heading down a bad path. If he is not willing to accept or acknowledge this, there is only trouble in the future.

I like the suggestion to make your presence known at school. If this girl doesn't come around and say, have lunch with you guys, then there is a real problem. Come for lunch everyday, stop by as he is getting ready to go home. Check in and bring a snack when he is studying late hours right before exams.

You need to spend time together as a couple. In first year, he should be able to study like a 9-5 job and be home outside of those hours. If not, perhaps you should take up reading and go with him to the library at night, or start doing research on an area of medicine that you find interesting. Help him study by quizzing him on the material (esp. since you will be going through it in a year).

There are a good number of married people in our class, and they do go out, but as a couple. Not alone, and never for a drink with someone of the opposite sex. The fact that you weren't invited/didn't go along is a huge red flag.

Starting medical school is somewhat like high school all over again- smaller class sizes, everyone has their core people, "groups"- although a little different based on what people want to go into or similar backgrounds, gosh, we even have the freaking lockers. He may be struggling to find his place in the class with mostly younger people- he needs to be comfortable with who he is, and find a niche that doesn't jeopardize your marriage or his reputation (as I said, I am sure people are talking).

Good luck, and enjoy the time off before starting in the fall!

Sorry I agree with path. Even though I trust my husband and know he means well, I also know he is a male and given the right circumstances could be tempted to stray. I think this is just the way it is. So it is important to work on maintaining our relationship so this will not happen (infidelity). Because once it does happen, I think it is so much harder to deal with. And might even be the end of our marriage. So preventing it is so important!