Did Finland's border authorities make a mistake?

I live in Denmark, my friend comes from Korea. Last year on December 21st, she (Korea) travelled here with a transfer in Helsinki airport. She's staying here until January 21st (31 days). Now, last summer she was in Europe for 77 days from July 1st until September 15 (77 days). There's 97 days between September 15 and December 21, so there shouldn't be any problems entering Schengen according to the 90/180 days rule.

Anyway, when she arrived in Finland on December 21, she was told to be back in Finland by December 31/January 1st (Just any day, which already here sounds weird to me). This surprised me a lot, so we immediately went to the police station at the airport in Copenhagen, they looked at her passport's enter/exit stamps from summer and said they couldn't see any problems with fulfilling her stay throughout January and that the border guard in Finland had probably made a mistake. They (now there was two, because they were also really puzzled by this at the station, heh) then referred me to the beautiful Schengen calculator and it tells me there's no problems with her stay.

Now, I've printed out the results from the calculator and the documentation describing how it works as well as the rules from ec.europa.eu and I'm hoping she can use it to help her case, if they pick her out again when she goes home.

As far as I can understand, it's different case to case and person to person how stuff like this is handled at the border, but should we consider booking her another flight through another country instead of Finland (like, Dubai) just to be safe? She's not nearly as fluent in English as I am and would have a hard time arguing her case if they pick her out again on the way back.

UPDATE [January 21 '15): So we decided not to change anything, so she flew from Copenhagen to Helsinki where she would transfer, and everything went smoothly so it definitely was a mistake.