GagaJo I do not think the suggestion is that Covid-19 is only transmitted through social gatherings but that social gatherings are the way it can be spread to a lot of people all at once, who can then, each, of them pass it on to other people.
I believe the take off point for the virus in both Italy and South Korea where church congregations where 100 plus people could be infected in one event. If they just went to work and home and infected 5 people, which I think is the average, the virus could go from 0 to 100 to 500 cases within a week, after that it is in the population and contact tracing is impossible.
What precautions did your friend take when she went to the supermarket? Was she wearing gloves? a full length coat? that could be hung out to air once she got home, a mask, there are lots of arguments pro and con maskes, but even a scarf around your face offers some protection.