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One of two things is happening, either the carburetor is stuck open or the timerbase under the flywheel is stuck at full advance.
Fixing it will depend on what year the engine is.
If it is a cable type have him remove the ball and socket, then with the engine off manually move the linkage to idle. The cable in the handle could have broken stuck at wide open.
If he has the direct linkage then he will need to remove the link arm.
If you can get me the year I can help more. Model number would be great.