If it is a fracture then they are right. Rest is indicated. Binding the ribs with a supportive bandage helps in acute pain with breathing.

I find a simple rehab tool for a fractured rib is to blow up balloons. Go by a packet and get those lungs working against that rib cage.

Please note that a fractured rib can and often does load to a pneumothorax. If this is a possibility then see your GP and possibly get a pain chest x-ray.