'Stand your ground' is in effect. However, 'No Trespassing' signs are useless. The police don't answer those calls. Why? I don't know. That's what the cop told my Mom.
I don't recall what Foodlion said...however do me a solid and put me in check if I start to go down his (or her) path.
I think the no trespassing signs make it easier for the cops to make the people leave. When I've reported trespassing, that's the first recommendation. Not because it does any good, but because then the cops can tell the trespassers that they were warned.
As you probably know (but your mom might not), sometimes cops have a higher priority happening, so they can't come. It's not that they wouldn't.
I would try to sabotage whatever benefit they get by trespassing. If they get privacy, prune back the landscaping. If they sleep, install bright lights on intermittent timers. Perhaps a sprinkler would be a deterrant? If they congregate in an open space, she could fill the space with a vehicle or trailer. Sounds awful.