Hard-core diet change should bear results very quickly (maybe within a couple of days) as the body rids itself of all of that garbage in his bloodstream.
Your dad needs to stricly keep away from fried foods, period. Next the processed snack foods (potato/corn chips, cookies) any paistries, etc. These foods carry the bulk of BAD FATS that so many people gulp down on a daily basis. 1-2% lowfat or skim milk also if that applies.
Though you should always consult your doctor, I say the hell with the 'lifetime' medication bullsh*t. That translates to *fat* profits for him and those drug companies. Most people REFUSE to change their diet out of stupidity or stuborness and hence they'd rather swallow lots of expensive pills.
Those pills only address the symptoms he now suffers- not the cause of them- a poor diet.