A resting heart rate below 50 is called bradycardia.
This does not necessarily mean that there is a problem. A lot of healthy people have a <50 heart rate.
As long as the person is feeling ok (no dizzyness, fainting, extremely tiredness), there is often no problem.
Bradycardia can however be a serious problem if the heart doesn't pump enough oxygen-rich blood to the body.
How is your dad feeling with this low heart rate? What is a normal heart rate for him?
If he is experiencing dizzyness, fainting, extremely tiredness or chest discomfort you should contact his doctor/cardiologist.