He may have set the float level to high or his Nitrophyl float is heavy and is causing the main body to over fill. The Monojet is not prone to warpage like its older brother (the "B"), but it still can if screws are over tightened. If it is a warp problem he can try two gaskets or lightly coat the sealing surface of the gasket (both sides) with a light coat of Permatex #3(aviation form-a-gasket). It is in most parts stores in a small can w/brush. Its unaffected by gas or oil.
Chances are it's the main discharge. Make sure there is a check ball in the discharge hole and a spring on top of this. If it's all there, then using a brass drift punch, tap on the check ball to seat it into the hole. Make sure the check ball is the bigger check ball. Should be about 1/8" diameter.
The main discharge has the T, which holds the check ball down.
Test by filling the bowl with mineral spirits (on the bench), hold the check ball down and pump the gas. Gas should not leak by the check ball and out the main discharge.
The wrong gas cap (unvented) can cause pressure in the gas tank forcing gas into the carburetor and out the discharge.
Some carburetor have a vent tube leading out of the top of the float bowl and into the PCV valve. It cannot be plugged. Earlier carburetor vented to the air. Be sure this vent isn't obstructed.
There is a detailed article here: What is Flooding and How to Fix it