My wife is about the same size as yours. I picked-up an essentially NIB used Beretta 20GA AL391 with the 24" barrel and youth stock (in the real wood, not the newer XTRA wood, which she appreciated looks-wise) from the Cabela's Gun Library for $750. She is not very strong, however she has no trouble shooting this...actually quite good at sporting clays with it. I put the Briley Bolt Handle and EZ Bolt Release on it which makes it much easier for with her small fingers to load.
Here's something similar at Cabela's now:
http://www.cabelas.com/product/Beret...tta+al391+20ga