Like others have said, it's not that the hoof has to be trimmed differently, it's that the hoof is just shaped differently. Like Cherie said, on average a normal mule foot will look very similar to a clubbed horse foot. But, like another poster said, if he is sensible about trimming the hoof the way the hoof should be trimmed to maintain balance, then the mule's hooves should tell him how they need to be trimmed.
IMHO, if you trust him, go ahead and let him do a bit of reading and then give it a shot. A good farrier should be able to pretty well adjust automatically once they figure out the basic differences between the 2 hoof shapes.
That's been my experience with good farriers anyway LOL.