To many assumptions are being made here. You cannot extrapolate gallons from the MultiFunction Display (MFD) with any accuracy. The MFD is a rough indicator of current Miles per Gallon. In my Prius II, the MFD is off by plus or minus 15%, which is huge. Most of the time it is within 5-7%. Once it displayed 25% higher than actual (for an entire tank).

The only accurate way to measure mileage is to fill (and top off) the tank at the same gas station and pump at every fill up, then do the math Miles driven/gallon used.

Last time I filled up, and drove home from the station, and I recorded 99.9 MPG on the display (a little over 2 miles), but the next morning when I left for work, it immediately dropped to 54 MPG (in a block or two).

Have your friend log his tank data in the Compare section on this site.