Beet juice helps lower blood pressure. Athletes even drink it to improve oxygenation of their blood before events. It is harmless, just need maybe 4 ounces a day. I use it.
Celery also helps lower blood pressure, I eat a few sticks a day, with a little peanut butter but humus or other toppings are good.
Pineapple juice has lots of sugar. Your dad should avoid sugar.