I've been reading this fic in Russian for a non-Russian non-English fandom. And someone called their friend's mother Mrs.
We don't do honorfics it's not exactly a thing in modern language. You would call someone "N's mom" or "Name Patronym" or "Aunt M" if you are close enough.
For translation purposes usually used are own honorifics but they are a bit archaic.
Language of fandom in question also has their own widely used honorfics.
It's been a day but I am still so annoyed.

Anyway. Feel free to vent about your fic grievances.