Lots of variables will I suspect come into play here. Firstly, as potshotpat says, report it ASAP if he hasn't. The consequences of it being used in crime and being recovered by the police before he reports it when he could have earlier would be bad!
The other things that might come into play are, as Oracle says the arrangements he made for safekeeping/security. Could you see it in the pickup? How long was it left unattended? What sort of area is it (known for thefts from vehicles or theft of guns)? Were there any indications there might be a gun in his vehicle (BASC stickers, pheasants on the back seat etc.)? Was it in the back of the pickup which is not alarmed, where the passenger areas are? There's probably more. But it would be worthwhile your pal getting his ducks in a row before they speak to him about it.