Check the oil and make sure its not overfilled. Either he or you may have tilted up the mower with the carb side down which can flood the intake tract with oil. Frequently this will 'hydro-lock' the engine to where you can't turn it over. Pull the spark plug and ground the wire - pull the engine through several cord pulls to blow the oil out of the cylinder. Check the air filter - a paper filter will usually be oil-soaked and should be replaced. A foam filter should be cleaned and re-oiled, or at least squeezed out to remove excess oil. The engine will smoke for several minutes after starting as the oil burns out of the works.

My MTD edger/trencher has similar symptoms as the above poster , but gas is leaking out of the muffler not oil. Has the metal gas tank with the plastic carb pickup tube mounted on top of the tank. Is there a simple fix for this? The engine is a 4.5hp B&S.