does your dad use all 4 edges effectively throughout turns?
do his tips tend to roll over?
does he keep his skis on a parallel yet non-linear course?
just tell him you have no idea what this means (even if you do know) so that it doesnt matter what width ski your on because you won't notice and neither will he.
and tell him my everyday ski is 92 undefoot, even though im on the west.