My girlfriend is 25 and she never had period. She also complain about not being able to use iud. She's experiencing stomach pain. What should she do?

Never? If she really never had a period she has primary amenoerhea and she may have serious difficulty to conceive. If she had menstrual periods before but not recently is called secondary amenorrhea, first rule out pregnancy second other conditions. She needs to see a gynecologist to find why and then decide what to do to administer her fertility.
Doctor can evaluate. A person learns more and more about a girlfriend or boyfriend as the relationship progresses. In the U.S., where almost every child has access to medical care, a girl at age 15 years will have seen a doctor for not having periods. This is because "all" the other girls she knows at home, school, or church have started their periods by that age. She must see a doctor, if for some reason she has not.