@OverSky in this case, Emilyko is explicitly referring to Rosemary, so using "their face" would be unnatural. In a sense, Emilyko knows who it is, but the reader doesn't, so a large part of it was trying to convey this while working with the fact that you need a subject for a sentence to be grammatically correct in English.
One thing though is that while a Japanese reader would get the double meaning, they wouldn't know the gender of the perpetrator, so this TL kind of gives away one more hint than the original but in turn make the other more vague.