Low Blood pressure evenings

For the last 3 months my friend has been experiencing a headache (2pm 7pm) , some times mild other times very strong. After hearing about his problems I suggested taking blood pressure several times a day. His blood pressure in the morning and at mid day is an average of 111/71 and today in the evening his bp was 84/67 with a strong headache. He is not taking any medication except Advil for the headaches. He also is complaining of being tired. He does have a slight heart valve problem and recently diagnosed with a genetic eye problem. He was prescribed glasses (thought to be original headache problem). We are looking for some reason for the headaches and a fix.