If your friend only has one beer or one glass of wine, he should be more than fine. The main idea is that he doesn't drink a bunch because of any pain medications or other pharma he might be on. A little bit of alcohol won't hurt. If he can keep it in moderation, he'll be fine.
I would make sure there are plenty of non-alcoholic beverages on hand though and that he isn't the only one having some of them. It will help keep the "peer pressure" down.