The real problem is that he didn’t attempt to solve the problem. From the sounds of it I doubt he didn’t get the job because he didn’t know the answer but because he didn’t try. When I’m interviewing someone I am not looking for them to know the answer I’m looking for how they attempt to solve the problem. Do they ask questions to clarify? Do the use available tools? Do they ask for help? Coding isn’t a solitary activity resources are available there is nothing wrong with saying “I don’t know this”. There is something seriously wrong with saying “And that’s that so I give up”