Yes. L series are vented via a center hole. Welcome.
Two things. Make sure he has the correct cap since it appears that model should have a vented cap. And the other thing is make sure the choke is closing completely. If that mower has the single throttle/choke control there is a good chance that the throttle cable has slipped in the clamp at the front of the engine and is now not completely closing the choke.So what should Mr. Barnes do ?