I wouldn't do shit. I'd meet the guy. Having been an 18 year-old in the past six or so years, and having a brother who is currently 18 (w/a 15 y/o girlfriend, mind you) I can tell you with absolute certitude that 18 year-old boys have their heads in such a position as to preclude rational thought, namely, between their ass-cheeks. This can be clearly seen in their interest in a 14 year-old girl, one of the flightiest, most fickle creatures God created. They're adults in legal terms only. So I wouldn't treat him any different than any other boy she brought home. Hell, I might just prefer him to others: the only thing dumber than an 18 year-old boy is a 16 year-old boy, because at least it can be said about an 18 year-old that he has a head, however ass-bound it is. A 16 year-old boy's closest analogue in nature is a recently-decapitated chicken.
CLIFF'S NOTES: All you guys screaming statutory rape need to loosen the fuck up remember what it was like to be a kid. I don't know if you guys remember your 18th birthday, but I do. I didn't get some magical fucking revelation, some divine insight into what it meant to be an adult, my dad didn't pull me aside and hand me my official Man Card, or Manual of Being a Man, or anything like that. There was no transition. It was just another sunrise followed by another sunset. I was just as confused, stupid, and horny as I was the day, the month, the year before. Felony charges and a permanent record are no way to teach a boy not to think with his balls first time around. At most give the kid a talking-to about the kind of legislation he's up against. Statutory rape laws were never intended to punish boys for dating girls who go to the same school.
EDIT: This isn't to say that if it were my daughter, I wouldn't let him see me cleaning the guns. Of course I would, but in this respect, I wouldn't treat him any different than any other boy, 14 to 17.
EDIT to the EDIT: Just to be clear, I'm not expounding on how to deal with girls bringing boys home. I haven't figured that part out yet. I'm merely saying that treating a boy differently than others simply because of some ridiculous law that supposes he's an adult is childishly vindictive. Hell, I'm pretty sure I'll kill and dissolve every boy any daughter I might have brings home. It's just that the 18 year-olds wouldn't be treated any different because of their age.