We have a rule that checks the desired forum username against WoW Armory to see if such a character really exists. (This cut our spambot registrations down to zero.) So if the character has an "é" in their name, then for registration purposes the forum username is also expected also have it.
However, passing around interesting characters like "é" might cause funky codepage conversion issues. That is a bit hard to debug without knowing the name in question. So Serendipity, if you don't want to add the name here for privacy concerns, you could PM it to me.
Alternatively, I assume the person used accented characters in their name because the version of the name with unaccented characters was already taken. In theory they should also be able to also register with the unaccented name. So if their name is for example "Johndoé" then they might be able to register on the forum as "Johndoe".