If the skin is not broken she should be fine. If its broken then they usually give a tetanus injection at your GP's surgery, its not an A&E job. Same as if you cut yourself on glass or a rusty nail when outside somewhere. I think children can have a tetanus jab also, but thought it was now given as standard in one of the multi vaccinations when small.
I'd bathe the area in antiseptic is all.
I've had a few nips over the years (not the dog being aggressive, just grabbing the ball same time as me, that sort of thing) and the skin is not broken, just reddened, its ok.
What I am wondering is how the dog came to nip her on her foot like that? Something terrier sized I am thinking as the bite spread is not large. Many dogs nip and don't break the skin, and often they are not attacking a person, its accidental. I won't judge your uncles dog as I don't know the circumstances.