Fired from job because took a rapid test and couldn't get a pcr test

Hi there, I'm asking for a friend here.

She was working as a Healthcare aide for a small private company. Since she works with vulnerable people (homecare) anytime she feels any fever or discomfort she gets rapid test just to make sure she doesn't get others sick. So she took a rapid test 3 days ago and it shows she was positive. She informed her employer who said okay , we will cover your shift. Please go get a pcr test done so we can know the strain. She tried to book an appointment but because she already is positive they said stay in isolation for 14 days. After telling her employer , 3 days later she calls back and says your fired because you failed to take a pcr test which is protocol. Since you went against protocol your fired.

Is that legally possible ? Could she fire her on this ground ? What rules/laws protect her . Please advise