Wow, big contrast. I would had thought the back of her leg slate/black from the first photo. Many barred rock hens tend to have splashes of dark on their legs, even going all the way down to the foot. I don't know that I've ever seen as an extreme version as this hen but it could be that. I bought a local flock once from a guy who had easter eggers and one turned out to be a cockerel that looked pretty much like a barred rock (with flaws in regards to SOP obviously). I'd still wager some sort of mixed breed or that his stock is basic hatchery stock, which realistically is the same thing as a mix breed (to a degree).
If he hatches his own chicks, and you know him - I'd make an inquiry of what breeds he has in his flock (if it's that important to you); if not, then I'd still call her a barred rock; as mentioned, there isn't much difference in regards to most hatchery stock birds and that's what people commonly expect..