He will probably find the vent tube pinched. I found this out the hard way on a long road trip after going through some mountain passes. And he needs to address it asap as he can potentially end up with a crack in the tank causing fuel to pour onto a hot engine or exhaust. For me, it happened after I had removed the fuel tank to upgrade rear spring. I didnt pay close enough attention and when reinstalling the tank, accidentally pinched the vent tube hose. The vent tube hose attaches to front right on top of fuel tank. It then runs to the left and back to right just to the rear of the air box and then down to the charcoal canister mounted in front of lower part of motor.