O/U has a much smaller selection thats under $2k and still considered decent quality. You can get into a base Browning Citori for under $1500, and the base model Berettas run just under $2k. Stay away from the cheaper O/U's like the mossberg or Browning Cynergy- its better to buy a used gun or a semi-auto at that point.
I would not compare a Browning Cynergy to a Mossberg. The Cynergy may not be appreciated by purists but it is one smooth swinging shotgun. Not to mention, the gun might fit you better due to the lower receiver height of the Cynergy. Instead of looking into the back of a traditional receiver, you would look over the top of the Cynergy receiver. I love my Citori's but also really enjoy shooting the Cynergy.
Just remember...fit is the most important. Hopefully you can find a gun that will fit both of you but that may be a challenge due to the differences in male and female bodies. If recoil may be an issue for either of you, you may want to stick with a semi-auto. Recoil from an o/u will hit harder unless the gun is weighted which may tire you out from holding a heavy gun. There are lots of things to think about.