As far as we know, they only went to the Yule Ball together three years and a half before he died. There is no proof they ever dated properly (Harry and Parvati never did, for example), and even if they did, three years and a half is long enough to break up somewhere in the process. And actually, I must say that I think this idea that they were together when he died is kind of offending towards Fred: do you really believe he would have tried to seduce Fleur’s Veela cousins at her wedding if he was not single?
(my own theory is that George has loved her all along and was too shy to ask her to the Yule Ball so he asked his brother to take her there so she would not go with someone he didn’t like)

