User blog comment:PitaKang/FANFIC! :D Muggles vs Wizards!!!!/@comment-4207136-20120825080406/@comment-1720115-20120825171630

Fiendfyre can go out. Hermione mentions in the last book that you can control it, but it is very, very difficult to do so, which is the reason why she never suggested to use it for the Horcruxes. Also, I doubt Malfoy would have been able to gather enough control over himself to grab the diadem while the fire is surrounding him.