Aha, this complicates things somewhat.
Basically, unless she has confirmation of the renouncing from Poland, the country will not regard her as having actually done so. All the information is here - polish consulate ny/en/m.22.Polish_Citizenship.html
If she merely renounced it to the American authorities - then she's still a Polish citizen and cannot be given a visa. I would say that it's almost certain that they will refuse to give her a visa - but equally so, she cannot be punished for overstaying the 90 days stamp, because she's already a Polish citizen.
In this case, it's really simple - she should go to Poland and obtain a "dowod osobisty" within 90 days. It's rather simple for a Polish citizen to acquire. The good news - if she gets it, then you'll be able to get Polish citizenship too - which will allow you to live/work in the EU freely!
I knew a couple of Americans who live there (and illegally work) without visas.
It's madness to try and do that now - I can confirm that on the Polish/Ukrainian border, they are checking documents very, very closely. Even me, with a British passport, had it checked quite carefully because of the presence of quite a few Schengen stamps (I try to collect them where I can :/). As far as I know, legally, the presence of a stamp doesn't actually confirm residency (unlike in Ukraine for instance) - it's merely an aid for the passport holder to know when they entered/left the zone.