I your Dad's rifle actually hits point-of-aim at 50 yards, and then the next shot is off by 10 inches, all I can say is "WOW!!!"
That kind of wild inaccuracy is absolutely caused by something simple: either your rifle"s
1. buttstock is loose (possible),
2. sight screws/scope mount bases are loose(likelier),
3. defective telescope (likely), or
4. barrel is defective with buldged bore, damaged crown (least likely)
Try inspecting the bore carefully with a flashlight after a good cleaning. A buldge or bad crown is easy to spot. Loose scope wounts or bad scope is easy too. A good .308 Winchester rifle should place bullet holes close together, almost touching, at 50 yards.