Step-by-step explanation:
The consciously recalled explicit or declarative memories are processed in the hippocampus in our brain, and the implicit or the non-declarative memories of procedures and skills are processed in other parts, including the basal ganglia and the cerebellum of our brain. These are used to play games like the checkers.
In the context, our friend's father might have damaged that part of the brain which processes explicit memories and not those parts that process the implicit memories.