I'm not sure about any difference between employing someone and paying for services of someone who is self employed and how the authorities would see it but it she is found as working for the company, there can be high penalties (more info here
http://www.ukba.homeo...ng-illegal-working/). There can be a civil penalty of up to £10,000 per illegal worker.
All it takes is one tip off from anyone, evener getting questioned and found out say in and out of immigration control say if she goes out of the country then gets stopped and questioned coming back in at an airport/port (I think I've been watching too much many customs programmes!).
Sums like that could ruin a company or at least severely impact on finances and potentially other jobs.
Can she not get a visa/sponser etc... through the proper chan