Is there a device to help an elderly person hold and turn a key?

My father ordered something like that, something like a small grabber, which helps him hold onto things. It was from some mail order catalogue; I don't know which one. But Blannie's suggestion is a good one; searching online could help you find one.

Another possibiity is to find a way to attach something metallic, then use a small hand held magnet to grasp it. I haven't tried it though, and offhand other than using Gorilla glue, I'm not sure how well it could be attached or would adhere to a metal key.

Google "key turners" and you can find some assistive devices that are designed to make it easier for someone to hold a key and apply pressure to it.