if she's 18, it's not criminal, even if she is a student ... the "person of authority" clause applies only to minors ... as far as the law is concerned, as long as she's 18, it's no different than a co-ed and her professor
however, the rules for teachers go further than the law
During investigation, it could actually be proven as criminal. The girl is 18 now, but has she been 18 the entire time that he has known her? Unless he, or she, has just recently arrived at that school, odds are that he knew her as a minor and could have taught her or had contact with her. The media LOVES this kind of thing and the School Boards and Teachers' Union love to make examples of people that even walk this line. He would stand a very good chance of losing his job and getting charged criminally with sexual interference because, as others have mentioned, it would be considered grooming.
He should delete her from facebook completely and drop it. If he runs into her by chance after she leaves the school or reconnects with her through facebook after she's long gone (not hard to search her when he has her name) and they hook up, that is different. Still kinda wrong though. Dating an 18, 19, 20 year old when you're in your early 30's is not necessarily wrong, but when you are in a position of trust, such as a teacher, spiritual leader, counselor, neighbour, etc, it is. If he wants to date younguns, tell him to hit the University/College bars around town and pick up some fresh young meat that is just out of high school to quench that thirst.