From what I've read, the vaccine doesn't guarantee you don't get infected. If infected after being vaccinated, it is supposed to reduce the severity. It seems like he could be asymptomatic with the original strain. Should be interesting to see what they find through further testing on your friend.
https://www.forbes.com/sites/coronavirusfrontlines/2021/02/26/could-vaccination-cause-me-to-test-positive-for-covid-19/?sh=56c63f8b38b3