Has she had a bone-density scan? It does sound like osteoporosis, but x-rays and ultrasounds are not the way to diagnose it, she should have a scan specifically to look at how much strength is left in her bones. If her regular doctor has not ordered a bone-density scan, her ob/gyn certainly will. (I've had two of them, and I'm younger than your mom, both ordered by my gynecologist.) This will help to rule out (or rule in) the doctor's idea that her bones are "bowing out." And if they are, she should be told if they are at risk of fracture. It sounds logical from the height loss that she has weakened bones, but it doesn't sound like the doc has checked to see her painful rib is suffering from microfractures.
Is she taking a medication to strengthen her bones, such as Fosamax? Is she taking extra calcium? The doc should have prescribed one of those or both, plus exercise, if he was sure she has osteoporosis.
If the bone-density scan reveals that she *doesn't* have particularly weakened bones, she should see a different doc about her back and what might be compressing her discs.