Friend tried using garlic rubbing on the skin for扁平疣.

Are you sure they are 扁平疣? If just 疣 people apply castor oil (from whole food) on the skin and they will fall off after a while. They are virus so be careful.
Watch what you eat recently. If you had foods make your body more acidic (sugar, cake, too much protein, etc) you will see this kind of problem more often.