If your friend is immunocompromised then he/she is taking risks being in close contact with others without masking or other steps. I have a few friends that caught covid on flights, so any place where one is close to others poses some risks. When someone gets sick, the virus or bacteria invades their cells, multiplies, and spreads until counteracted by antibiotics, antivirals, and/or ones immune system. Transmission to others by touch, sneezing, cough, talking, or whatever can happen regardless of the strength of that infected person's immune system. Your risk of catching their cold, flu, or other is lessened by waiting a few days to ensure they didn't catch anything from the nightclub, by keeping up on vaccines, keeping ones distance (across a small table), masking, and/or hopefully being immune to whatever pestilence is about because you already were sick from it.