First, he doesn't need your approval to do it.
Second, he's been sober for a year and a half--he didn't just put the plug in the jug 30 or 90 days ago.
Third, if this is a good friend getting married, I can understand why he doesn't want to be left out of the festivities.
Fourth, there are AA meetings in Costa Rica. I was just in a heavy drinking environment for a week on a business trip, and when it started getting to me I skipped dinner one night and went to a meeting instead.
If one is going for the right reasons (and it doesn't sound as if he likes to hang around bars for the sake of hanging around bars), and he has a good plan in place, and he feels like his head is in the right place, then I see no reason why you should discourage him from going.
My two cents as an alcoholic sober four and a half years.