My father is 62 years old and he complains of strongly bitter mouth at night during sleep. What is the diagnosis and treatment?

Could be reflux. If it's just happening while he's sleeping, then there's possibly a small amount of stomach contents going back into the throat due to gravity. Even a very small amount of reflux could give a strange taste in the mouth. Advise him to have no food or drink at least 3 hours before going to bed, and avoid alcohol at night which could make it worse. Best of luck to your father!
Reflux of bile? Some medications can cause bitter taste in mouth, but you did not mention any. Usually acid tastes sour and base is bitter..Stomach contains acid, but bile is basic which is bitter..If he has some bile that is refluxed all the way up to his mouth, bitterness is perceived. There isn't anything else in the body that is naturally bitter. Consult doc.. Good luck.