Why use Mach number at high altitude and not IAS
May I suggest your friend could be satisfied by you saying that for aircraft types not specifically designed to fly at supersonic speeds:
There is a Mach number above which you may loose control (very often in pitch but can include roll).
There is a Mach number above which your range performance drops dramatically.
Further more:
At low levels you always reach IAS limits before Mach ones
At high levels you always reach Mach limits before IAS ones.
PS
An explanation of these simple facts is another matter entirely.