Thanks to all who replied.
It's a business, and I don't know what quality receptacle they have. It is a good idea to try other receptacles to see if it stays. However, I suspect, like many of you, she won't be able to convince the owner to change out the receptacles. In fact, I don't think she would even try. Although I do most electrical work (replace switches, receptacles, etc) at home, I'm not going to rewire their stuff.
The electrical outlet is right under a counter. Here's a sketch:
Some of the ideas posted may help. If she doesn't mind an extension cord, I could try to find a 3-prong extension cord with a firm gripping receptacle end.
The hospital grade receptacles is a good suggestion, not necessarily to fix her problem, but a good thing to know in general. I see there are also hospital grade extension cord, but the plug seems to take up alot of space. Is there a local chain stores I can walk in to check these out or are they mail order only?
Another idea is to zip tie a metal ring a foot away from the plug and hang the ring on a screw-in hook on the wall. The hook would absorb the cord weight and movements yet it is easy to unplug and move to another station.