If a freelancer's health is a risk to other freelancers, staff and customers, you are within your rights to send them home and report it as 'gig not completed'. If the freelancer becomes unwell during the gig, you can send them home and only approve the hours they worked.
Our freelancers are dedicated and may still want to work when they are unwell. In this case, we encourage you to come to an agreement. Suggest matching with them for a future gig when they are feeling better.