Originally Posted by
Yoshi212
That is true but some modes of transportation and some countries are less stringent about this. So a trip to turkey and flying out to the US from there without entering the EU again could save them the hassles and possibly some money.
How do they get to Turkey without exiting the Schengen zone?
The other way around but presumably the same rules:
http://www.feribot.net/feribot/?partnerid=Mzg=&lang=en
Note that when you sail from Turkey to Greece, even for a one day excursion, you will pass border control and enter the European Union !
Wherever they exit, and wherever they exit to, it will be found that they've overstayed. Unless they leave Greece/enter Turkey illegally, which leads to all sorts of other problems, including the fact that their exit from the EU won't be recorded, so the next time they try to visit they'll have problems.
OP, if it were me, I would advise them to (a) leave as soon as possible, admitting that they've only realized their mistake but are making an effort to fix it right away, and (b) try to leave directly from Greece to a non-Schengen point so that they can have this difficult conversation in a language and setting where your MIL is comfortable.