You also need it to establish your credit to be able to get a loan to purchase more expensive items later such as a house. Tell her to get one with a low credit limit so it doesn't get out of hand. If the issuer raises the limit later tell her to call them to lower it back. It's also good for emergencies.
Oh trust me, I know. Even though we live together, we keep money separate. I pay everything with cc. The ease of using my chase card and immediately transferring the money from my checking.
The increased credit score and free money makes it almost impossible for a cheap ass not to