It's a University, there's room for more specialized courses, Defense Against the Dark Arts is quite broad.
My guy is probably working on his thesis or some other big project for his degree, and he's going to do that by developing this course.
Fighting Fire with Fire, powerful and dangerous magic for use in the field against enemies of goodly wizards, be they monster or Dark Wizard (a militant specialty).
It's fine with me for him to be working under GF's character. I want to lead students on lots of dangerous field trips.
Or, if there are no other players who want their characters in my class, he can just do that in the background, and show up with all sorts of burn marks and bruises and trophies and stuff from his adventures.