How is her vision in that eye? Does she see double? Is the left lid droopy? Can you tell if the pupil reacts to light? Has she had any eye surgery? Does she have any eye pain?
The most common cause of dilated pupil is pharmacologic. That is she may have inadvertenly gotten something on her hands and then went to rub her eye. It could be household cleaners, fertilizer, insect repellent, etc. This would cause the pupil to be dilated (how long depends on the chemical and amount). This will eventually wear off without any long-lasting effects.
Sometimes, it is the pupil that is small that is the problem. There is a condition called Horner's Syndrome that causes one eye to be constricted. It can be caused by a variety of nerve problems. This would have to be diagnosed with testing by an eye doctor.
There is a condition called Adie's Pupil which is an inflammation of the nerves. This is also benign.
Other rare causes of one pupil dilated usually have many other neurologic problems associated with it.
It doesn't sound like an emergency; but she needs to see an eye doctor soon.
Hope this helps and thank you.