Of course first the person wearing the mask has to have the virus -- most people wearing masks don't have the virus, they're just wearing masks because it makes them feel safer (I've heard mixed opinions on whether masks are at all useful for the healthy. Medical professionals seem to discourage it because it's one of the main reasons for the shortage of masks for health professionals. They say if you have the virus a mask does help prevent the dissemination of the virus from coughing or sneezing, but unless you have a respirator mask an ordinary surgical mask of the sort most people are wearing doesn't actually prevent breathing in these particles very well so they're not very effective. But Chinese officials say they are necessary.) As for the rest, I guess theoretically if all atmospheric conditions are just right to get the flow from a sneeze or cough right to where you are standing and the mask is unsuccessful for some reason in stopping wide dissemination of particles, then maybe. Doesn't sound like something worth worrying about, though.
"For the average person not working in a hospital, Milton says the recommendation to stay 6 feet away from others sounds reasonable." Research is ongoing.