What should I do if my coworker was exposed to a positive case?

Your coworker should determine if they have had a close exposure, as defined by being within 6 feet of a person with COVID-19 for a prolonged period of time (defined variously as > 10 – 15 minutes), or direct contact with infectious secretions of COVID-19 case. If so, they should initiate “self-monitoring”, i.e. monitor their own health for fever and cough. You are now once removed from that scenario and “self monitoring” would be adequate. If you were to develop any symptoms, you should call your  doctor. Read more here.