As long as the assets dont contain code, they're kind of fair game. The rule of thumb is you cannot redistribute them, but if the person owns a legal copy you can point to them on their local system. It is not up to you to figure out if they're pointing to a pirated copy or a legitimate copy mind you.

I assume you, the player, have to provide the assets yourself, and the game won't run without them. Since the code does not contain the assets, there is no copyright infringement.