unlikely to be MRSA related. She may have something called Adie's Pupil, which is an abnormally sized pupil on one side (may actually be dilated OR constricted). It would be VERY unusual for an eye to dilate in response to bright light such as sunlight. the normal physiologic response is to constrict. No one wants to go to the doctor unnecessarily. But unless this problem is CLEARLY getting better quickly (which would suggest a pharmacologic cause, such as a medicine in the eye, as mentioned in my last post), I would still have her seen. I hope this helps you.
Please note that this forum is for general advice only - it is not a substitute for an eye exam by a licensed professional. It is not possible to accurately diagnose or treat any medical condition without a complete history and exam.