Should I mention this to my colleague?

My colleague recently (about a month ago) bought a used car. Interested in the milage it has done etc I decided to check it out on a popular car checking website. To my surprise I discovered that his car has no valid excise duty, and it hasn't had since May last year. Should I mention this to him or should I leave it? I don't really know the most tactful way to broach the subject.