Tell them you will take care of it. Take the opportunity to go through the cabinets and discard what's expired, donate what's unloved, and bag separately what you want to keep. It all goes to a separate part of the car (expired in trash). Next day bring back the stuff to keep and donate the other. Unfortunately, they won't remember that you did this. It's paranoia and is because the brain is broken.
Have him/her checked for a UTI. The first indication when my aunt had one was a man in her room, and music coming from wires in her wall. She unplugged everything. and could not be calmed by anything I told her. Urine showed physical signs of infection, so the doc gave her antibiotics. Test later came back negative, however, no more voices, music, people in her room, etc. after the 2nd antibiotic.
My MIL went though a spell ( well before she really went off the rails) when she thought everyone was trying to poison her. She would pack up her meal on a tray and take it to the bathroom with her if she had to go... No lie!!
You could "check" the food by tasting it first.