I am not suggesting they leave illegally but some routes of transportation and some countries are more relaxed about these things than others. Greece to Turkey may be more friendly(as in willing to overlook this) than Greece to Germany/other Schengen.
Originally Posted by
RadioGirl
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
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.