We don’t know much about Evan Rosier from canon. We know he was a DE, he took a chunk out of Moody’s nose, he died a year before the war ended, and he was Severus Snape’s friend. Not much to go on, but it actually really says a lot. Especially considering Evan died right at the time Snape turned on the DE. There’s plenty of room for the argument that the two are connected, and was in fact the basis of the one big fic I wrote with that pairing. With those circumstances, it can easily fit in with canon.