PSG = Prix St. George. You can look up the person who told you that she was a Grand Prix rider by doing an internet search. Her scores will come up, and sometimes even video. But you can also look on the USDF website for the scores as well.
No, the eventing dressage tests are different (although they test the same things). Very roughly- beginner novice to training eventing tests are like training to first level dressage. Prelim starts to touch on second level. Advanced tops out around a hard 3rd level test, maybe touching on some 4th level.
As to the person you met- who knows. There are plenty of people out there who have competed at GP.