If your friend got anything close to 50mpg, trust me it wasn't a Civic Si, it most likely was a Civic VX, that was the Honda fuel miser from 1992-1994 that was rated at 50mpg, an Si would never get 50mpg unless it was coasting down a long long mountain with the engine shut off.
FYI: 1992 Honda Civic Si's original window sticker showed 29mpg (city) and 36mpg (hwy)