Hi ******: Has your mom had an x-ray or mri of the ribs? I'm not familiar with pain that would originate in that area from stomach probs, but I'm sure there are others here who are. Also, if they think it is osteoporosis related, have they checked for fractures of that portion of her spine or ribs? Thoracic/lumbar/rib fractures can cause chest pain, and breathing probs.
It's not unusual to have rib pain after cancer treatment, but they would have to do either an x-ray or imaging scan to see what's going on with her ribs, to make sure that nothing has spread.
Good luck finding out what's going on...