One of my friends as been expiriencing blurry vision, she doesn't wear glasses or contacts at all. Her family just thinks her eye sight is just getting bad, but she says it comes and goes.
Is there some reason for this?

It could be related to blood pressure as well. She needs to see her doctor.

Or low iron. Or any number of things. Getting her vision checked wouldn't a bad idea, but if it's only occasionally, it's most likely something else.

No one can pinpoint what it really is because we cannot examine your friend's eyes over the internet. She needs to visit her eye doctor every year for an exam. Her doctor will be able to tell her what's going on.