The police in the USA can access the national database with driver's information from all 50 states. I do not believe that they can access information from other countries without going through Interpol, the FBI, the Homeland Security network or some other extraordinary measure. That is unlikely unless they suspect some criminal activity or terrorism.
The chances of her getting caught are probably very small, but should she get caught for some reason, the penalties could be quite severe. She would probably be suspected of terrorism and spend some time in jail while they checked with the UK authorities. When they find out that she is driving on a fake license, they would probaby make her pay some expensive fines, deport her from the USA and prevent her from ever being allowed to return to the USA,.