Why do people who have high blood pressure, get tablets to make them wee a lot.?

My friend had her blood pressure checked it was high so she has two b/p tablets a day and when she was going to dr's to get her b/p checked she took one of the diuretics the day before and one on the day of the appointment, her b/p is still high do these diuretic tablets really work. They are called furosemide, or could be frusemide, she cant remember name of them. If she takes one on the days when she gets her b/p checked will that lower it.