Your grandmother has possibly got her ribs fractured. The treatment is conservative.
Best thing would be to consult an orthopedician and follow his instructions.
If she is not willing for that, some of the measures which may help her are:
1. Avoidance of exertion
2. shallow breathing
in and out
3. Use of suitable rib belt
4. oral calcium and vit-D tablets
5. Oral analgesics.