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There are lots of different tests for malaria. You would need to determine exactly what type of test is used. Blood smears will test for active infection with the malaria parasite. This is for diagnosis. After treatment the person's blood smear will be negative.
Serology testing checks for antibodies to malaria. Once someone has had malaria, they will always test positive for malaria antibodies. That doesn't mean they will ever have symptoms though, they just have the antibodies in their systems.
If the person had malaria parasites in their system, the provider would have to treat them for it with antibiotics. That is the basic standard of care anywhere in the world.
You might consider asking for a doctor's report every time this person is sick verifying their ability to return to work.