warm-up throws by non-pitcher

[jodibuck]

In a varsity game today, while waiting for the pitcher to return to the mound between innings, the shortstop threw two pitches to the catcher from the mound. The opposing coach tells the plate umpire she has to pitch to the first batter of the inning, since she threw warm-up pitches. I have looked in the rules book and cannot find any rule to support this. And yes, the plate umpire required #6, who had previously been the shortstop, to pitch to the first batter.