Frozen shoulder is a condition characterised by pain and global restriction of movement. Most Probably your father is in the recovery phase of frozen shoulder. In recovery phase the patient regains movement and function over a period. The total duration of disability therefore lasts 12 to 42 months (30 months on average). Some patients may be left with some restriction of movement, but in most there is no residual impairment. Recurrence in the same shoulder has never been reported, though up to 20% of affected person develop the condition on the opposite side.
The best way to prevent frozen shoulder in the other arm is to contact your physiotherapist as soon as you develop shoulder pain that limits your range of motion. This will allow early treatment and help avoid stiffness, if possible. Also one should continue doing full range of shoulder movement and shoulder muscle stretches on a regular basis for both the shoulders. Your father should visit a physiotherapist who would guide him with home exercise program to follow up for his affected a swell as unaffected shoulder to avoid recurrence in other arm.