My mother was given a medication named progestin. What is this and how effective is it in the treatment of endometriosis?

Very. Depending on the amount of disease, progesterones may stunt the progression of the disease. Removal of the disease by laparoscopy may be needed if hormonal medication is unsatisfactory for the patient.
Can Be. There are several types of hormonal treatment for endometriosis, of which progestins are one type. Not every treatment works in every women and sometimes medicines becomes less effective over time.