If one of a couple just can't be left alone at home while the other shops, yes is is impossible to shop alone. There are a lot of Gransnetters in that situation. For others, one of them could stay in the car while the other shops, or they could take advantage of the many schemes to help the housebound and/or vulnerable.
They might even get a supermarket delivery if they re very lucky - and if they are internet savvy and prepared to sit up after midnight hoping they will be next in the slot queue before the slots all run out (I got a slot last night after sitting at the PC half-asleep until after 12.30, watching the screen refresh, and refresh, and refresh. It isn't until 22nd April, but Hey! its a delivery!)
Parents-in-Law. What do/did you call them?