Between innings warm up issue

[Andy]

This situation came up in a HS Varsity game yesterday:

Middle of the first inning, visiting team is taking the field on defense, pitcher is taking her warm up throws, coach comes out of the dugout with a bat and ball and begins hitting grounders to a substitute player in front of the dugout in foul territory.

I had never seen this before, neither my partner or I could think of any rule basis to not allow the coach to hit grounders to a sub player so we didn't say anything. The only time this happened was in the middle of the first inning. Any rule basis to not allow this practice?