Consider a 20 gauge for your girlfriend as it's one of the few gauges where light target loads are readily and inexpensively available (12 being the other). In a semi, like an 1100, it should kick less than a comparable 12 gauge.
Another option is a nice pump like a BPS, Wingmaster or Ithaca 37.
I would recommend you budget another $80 or so to have a smith install a really good quality recoil pad like a Pachmayr Decelerator 1" pad or a Remington R3. The OEM pads or plates on most guns leave a LOT to be desired. Most smiths will also slightly shorten the butt to fit her when they install the pad for no extra charge.
A good fit and a good pad will do more to make the gun comfortable to shoot than any other factor.
If she wants to use it for a lot of different activities, a model with interchangeable chokes might serve. For your budget, if you are buying new, take a look at an 870 Express 20 gauge here:
http://www.remington.com/shotguns/pump-action/model-870/model-870-express
This gun can be had with a compact 13" or 12" buttstock, as well as the standard 14" LOP stock. You can also get it in either 21", 26" or 28" barrels. I find female shooters often appreciate the 26" version as it's less muzzle weight for them to hoist.
If you plan to shorten the stock to fit her, get a wood stock. Synthetic stocks are hard to shorten.
Some people denigrate the express models, but realistically they work very well and do smooth out with use. The finish is not as nice as a Wingmaster (obviously), but both guns have the same bones. O the Express guns I've kept, racking the slide a few hundred times while watching TV helped immensely
