After the Triwizard Tournament, Fleur promised Harry she would be seeing him soon. In fact, this was her way of promising him her help.
Imagine Harry's surprise when he casts his Patronus, and the stag seems to run off. Moments after that, the radiant stag returns bearing a glowing rider on his back. Fleur Delacour, glowing like a vengeful star, rides into battle, wielding a bow and loosing gleaming silvery arrows at the dementors.