Fast Base Jump Airsoft Game Helmet tan [tb326] - $51.15 : Airsoft Shop
Something like that should fit. A few of the milsim guys in my area wear them with dye masks.
However, it should be noted that wearing a helmet is going to be really warm in combination with a mask. A mask with full head protection would probably be a better choice.
Also, unless there is a real need for hard shell head protection, she might do just as well to wear a padded head wrap, beanie, or watch cap with a little foam put in it.