Those who receive monoclonal antibody therapy should still get the second dose to be as protected as possible against COVID-19, but they should not get a vaccination while sick or during their isolation period to avoid spreading the virus to others. They can get the second dose as soon as their isolation is over, and it has been three or four weeks since their first dose - a minimum of 21 days for Comirnaty (Pfizer) or 28 days for Spikevax (Moderna).
Previous CDC guidance stated patients should defer COVID-19 vaccination for 90 days after monoclonal antibody therapy. CDC has revised this guidance based on encouraging evidence that patients who received these treatments still mounted a robust immune response to mRNA vaccination soon after treatment. Getting vaccinated is a priority, and a person should receive their second dose on schedule when possible.
For more information on the difference between isolation and quarantine, please visit https://www.cdc.gov/quarantine/index.html.