They will have to self-isolate. That means arranging accomodation (ideally extending the one they're staying at at the time of test), going there and staying there until the isolation period is over. That's all. Food should be arranged a deliverdelivery.(By the way, I really don't thing passports are flagged upon positive test. One, that would be too sophisticated, two, no one cares if an infected person leaves the country.)