Re: 300 win mag ruger.
My guess is that something came loose with your scope. You either have internals that have gone bad, loose bases, or something wrong with your rings. There is also the possibility that maybe he had taken the gun out of the stock since shooting it last and forgot to tighten down all the bolts (I've seen this happen).
The last thing that I've seen happen is shooting the wrong ammo. I know a guy who accidently ran some 25-06 ammo through his 270 and was super pissed that his gun shot like crap until he realized he had the wrong ammo! Is there any chance he was shooting some 7 mag ammo?
Also, did he shoot a .75 inch group and then they all opened up to over 10 inches? Or did it shoot well last year and this is the first problem he's had when taking his gun out this time?