How do I tell my friend she has oily hair?

Maybe you could offer to have a beauty day with her. You two could go get your hair done, nails, makeup, etc. Maybe you could say how great she looks and ask the hair stylist tips on what products to use and how often to wash your hair. After you're done, you could go out to eat or something. Maybe she'll get some looks from guys and...hopefully, she will see the difference in her looks. You could always say that she should do her hair like that everyday. If you are not able to go to a salon then offer to do each other's hair and makeup. That would be how I would try to help without directly telling her that her hair is gross.