I think by "Greek wedding" Maria meant the wedding of the future King Juan Carlos and Queen Sofia of Spain in 1962 and not Queen Anne's own wedding in 1948. Both weddings took place in Athens, Greece.
For the 1962 pre-wedding ball, Queen Anne of Romania definitely did not wear the Greek key tiara which was worn by her mother-in-law Queen Helen at the event but I'm not entirely sure what tiara she wore.
Option 1) Princess Irene, Duchess of Aosta's diamond tiara - Princess Irene was Queen Anne's husband's mother's sister
Option 2) Princess Viggo, Countess of Rosenborg's diamond spike tiara - Princess Viggo was Queen Anne's mother's brother's wife