How can one advise a sister who does not wear the hijab?

A Muslim woman must purify her intention for Allah and be mindful of the reward for guiding people, for the Prophet, peace and blessings be upon him, said: "For Allah to guide a single man through you is better for you than red camels." You should also read thoroughly about the hijab to be knowledgeable about the issue and understand why a woman might not adhere to it, so you can choose the appropriate means and time to invite her, whether through direct conversation or by gifting her a tape or a book.