Well...it would seem to me that the math wouldn't be hard at all. But getting numbers to do the math with would be a pain. You'd need to know things like how much force you applied to your pedal for how long in order to get such and such a distance. And you'd need numbers for how much force your friend applied to the ground in order to get so much distance.

It's hard to imagine that it would be worth the trouble. And I'll bet your friend knows this, too. He's probably just making an unprovable claim in order to bait you.

The next time he gives you a load like this just tell him, "Bicyclists are happier than walkers". If he disputes this, just keep on saying it.