He'll be fine! Cancun is very safe because it's FULL of American/Canadian tourists. As long as he doesn't get stupid, uses his common sense, etc. then there's nothing to worry about. Just keep your wits about you as you would at home!
I second the advice. He shoud be aware and alert and stay with his friends in a group. The alert indicates crime throughout Mexico, but the border towns are especially dangerous and have been for quite some time.
I'm in S Texas and we hear about the crime almost every day. We no longer go to Laredo, which is sad, because it's a great place to shop for pottery and iron and it was nice to stock up on Kahlua and Bailey's.
Last Friday (Feb 20), the US State Department issued a travel alert for US citizens traveling to Mexico. You can read it here: http://travel.state.gov/travel/cis_pa_tw/pa/pa_3028.html
I read online many US universities are publicizing this alert to their students since spring break is coming up.
I think it's smart for students to make sure they travel in groups, use common sense, and leave valuables at home or locked in a hotel safe and not carry large amounts of cash, their passports or anything like that with them.
Yes, just stay in the hotel zone or go to walmart with a group. Do not be loud drunk and dumb. Do not leave a club alone at night etc. Same rules as here use common sense.