In this case, it is safe to allow the nurse exposed to COVID-positive patients to use your mikveh, since she uses personal protection at work. As an additional precaution, she should take her temperature and check for any symptoms prior to arrival.
As an extra safety measure to prevent any potential asymptomatic spread, we recommend setting an appointment for her to be last on a given night, when no one else will be present, even in other preparation rooms.
Assuming your community does not yet have specific mikveh protocols for Corona, you may want to consult with your rabbi and local authorities about establishing them.
Kol hakavod on your efforts to keep women safe!