It turns out fleur and bill were never dating. After years of hearing about Harry from his parents and siblings Bill could not wait to meet him. When he finally did he realized that something was missing for the boy he had grown to somewhat care about through those letters with his family and that was love and companionship, and he realized that Ginny would not be that right fit for him or would even actually love harry for harry. During the Triwizard tournament Fleur started to develop feelings for Harry after much observation of him, especially after he saved her little sister. Those feelings were than more or less cemented after he saved her in the maze. But she was too nervous and afraid to approach him especially considering her comments regarding him after his name came out of the goblet. Add that on to the fact of her limited social interactions after her delayed onset of puberty and her Vella allure afterwards and she just couldn't tell him. Then when Bill started training Fleur he realized something was wrong , and so he asked her what was bothering her. Upon finding out about her feelings for Harry and her past he realized that she might just be what Harry needed. So he thought up a plan on how to help this two teens get together and his fake girlfriend plan began. They did not even need to interact much for it because it was just an easy excuse to get harry and Fleur close to one another. After multiple interactions Harry starts to develop feelings for Fleur Which was what Bill was hoping and on the lookout for so when it happened he knew. At said time he brought Harry to the side along with Fleur and let him know that they were not really dating and it was all just an excuse to help them get closer to one another. And so let the hilarity, the chaos, the improvements to harry, and the flowerpot commence.