S.W.
I think vitamin deficiencies might have more to do with hair loss rather than hair color change. Does either you or her father have a family history of going gray early? That will be your best indication.
My brother developed a tiny gray patch on the back left side of his head around that age (and by tiny, I mean no bigger than the size of a dime). He's had it ever since (and he's now in his 30s). His hair is blonde, so the patch was never very noticeable unless you were looking for it. And he never had any health problems. Sometimes I think our hairs just come out a little crazy - could be stress or diet or even environment related.