At the end of his sixth year, Harry’s Conjured Corporeal Patronus was so strong that it kept running for days before fading away. When it happened, it had already crossed the channels.
Fleur was not feeling well because she had just lost her little sister, killed by a dark wizard. She could not sleep for two days until a bright stag appeared in her garden — she knew what it was because she was a witch. It stayed with her until she could finally close her eyes.
She hoped that she would meet whomever the Patronus belonged to one day, it had given her the will to move on.
It was the Second Task of the Triwizard Tournament when Fleur found out who had Conjured the Patronus she saw last summer, it was Harry Potter, Hogwarts Head Boy and Triwizard Champion.
She watched him telling the bright stag to go find his hostage, after Summoning a broom.
After the task, her only goal would be to get to know him because he had unknowingly helped her. She had to thanks him.