The coil doesnt look secured, he probably could get two measurments right now easy. Remove the coil and run the blower and read the amp draw. Replace the coil and run it and read it again. At least you will know the change in load. The duct should not add much, but like you say the data is priceless.
I will take a guess that it won't add more than 5-10% of the draw he has presently (1/2 an amp at the most?). If it gets close to the motors max rating, I would step up to a little higher HP motor. They only run about $60-80 even for a 3/4 HP. I doubt it will be needed though.
e.alleg, what is the HP on your blower motor?