checking the end-all-be-all of calculators (
http://sheldonbrown.com/gears/), with 27" tires and 170mm cranks, 120rpms is (only) 38.6mph. But your friend is also going downhill, so depending on the hill, yeah, why wouldn't he reach 47mph, whether he pedals or not? In other words, 1) he's full of it if he thinks he's pedalling to get that speed, and 2) the huge hole in his declaration is "downhill".