If he came intoxicated to work, I believe that is a serious issue. The liver breaks down alcohol at a rate of about 0.015 per hour if memory serves me right, so at the beginning of his shift that could mean close to 0.1% in a healthy male. The other consequence is 1 hour later he may not have tested positive at all.
On the other hand, people with some kinds of diabetes have been known to test false positive in breathalyzer tests.
Last, for a healthy adult weighing around 160lbs it takes more than a full beer or a shot of 80 proof alcohol to develop 0.02. Sipping he wasn't.