Thank you for asking on HCM. In my opinion your friend may be suffering from severe depression or dementia. In aging decline occurs in virtually all the body systems including cognition but 66 years old people are generally able to perform the day to day activities, calculations and lead a normal life. If there is low mood, lack of interest in life activities, episodes of crying, feeling of guilt, lack of energy to do routine things, feelings of worthlessness, consider depression. In severe depression, ability to do simple calculations can also get impaired. The patient can also forget things in depression. If he is forgetting route to his home, his address, has tendency of getting lost, has impaired memory then consider dementia. I would suggest you to get him evaluated from a Psychiatrist so that proper diagnosis is made and treatment started in the right direction. I hope this helps you.