If this were my DD, I would tell the high school coach that she doesn't need a new pitching coach and if not going to him/her will affect playing time then perhaps the AD may need to be involved. I have always taken the stance of "if it aint broke, dont fix it." Good luck.
Some HS coaches will be open to "She doesn't need a new pitching coach." Others will not.
I'll tell you right now that if you tell the coach "She doesn't need a new pitching coach," it is my belief that odds are high that the coach will be less inclined to play her. Getting the AD involved is laughable, in my opinion. Odds are against you there as well.
"If it ain't broke..." From whose perspective?