Can 8-3 wire be used with a 10-50R 3 wire receptacle?
A NEMA 10-50R device does not have a grounding pin, so your father-in-law likely simply didn't connect the grounding conductor from the cable. A NEMA 14-50R, which should have been used, does have a grounding pin.  

Technically, you should have installed a 14-50 receptacle in the first place (to be code compliant). That being said. Loads of old homes have 10-50R devices in place, and have worked fine that way for many years. As it's currently wired, the range should continue to function without a problem. However, without a proper ground, there's always the potential for electric shock. It's up to you, whether or not you think the potential dangers are great enough to take action.