The frequency of a wave is 6 × 1015s1. Its wave number would be?

Your friend is correct.
wavelength = c / frequency ( c is speed of light in vacuum )
 wavelength = 3 x 10⁸ ms^-1 / 6 x 10¹⁵ s^-1 
                      = 5 x 10^-8 m

wave number = 1 / wavelength
                       = 1/ 5 x 10^-8 m
                       = 2 x 10⁷ m