The shouldn't coach of yelled, but some coaches do make it mandatory before the girls even step on the field. If that's the case then he does have the right to ask to put her mask on. I am pro mask and not just for safety reasons. I saw many girls improve their performance by wearing a mask (at many different levels), for example girls who have a tendency to turn their head to the side or pull up prematurely when a grounder is hit their way resulting in a ball under their glove.
Interesting.
DD 3 thinks that the mask makes her a better fielder, or at least a more self-confident fielder. She can make the play without worrying about whether the ball is going to hit her in the face. That may be one reason why the ball has never hit her mask -- she has always stuck out her glove and made the play in the nick of time -- so far.