The answer to that, Woofgang, is because she will be a Director of her own limited company.
This is a grey area and one that lawyers would love to make money from - arguing the two sides.
If you know enough people such that you are intending to work through another limited company that you are not a director of (and they just take a small handling margin) then it is worth a punt.
Personally I wouldn't set-up another new limited company of which I was a director to attempt this stunt (even if the economics/admin of setting-up the new company were small enough to justify it) - you would be too close personally to both organisations for comfort. You might get away with it because the agency might decide to do nothing. But then again you may find a legal letter on your doorstep.