If they came in super close contact with someone who is positive, they take the test now. Then, self-quarantine for 14 days, and need to be tested again after quarantine. Otherwise, then no re-testing is needed. For a concrete example, look up what happened with UP10TION.
Can the tests they use in South Korea detect the virus one day after exposure? Where I live, we're told to self-isolate for 4-5 days before getting tested or there is a high risk of a false negative. Either way, I hope they are all OK.