No credit card required
Browse credit cards from a variety of issuers to see if there's a better card for you.
Tell her to go to www.optoutprescreen.com and opt out. Adding her as an AU will not affect your credit. Only that one account will show on her CR.
If you add her as an AU, you always have the option of NOT providing her a card to use under the account. Dont give her access, as you assume responsibility for any debt she incurs.
Adding an AU can have serious consequences to the principal card holder unless they maintain control of the account.
As for debt collectors using AU status as authorization to contact another, no.
The FDCPA clearly restricts communication regarding a debt to the consumer. If she were your spouse, the FDCPA states that a spouse is also considered the consumer owing the debt, so they could make contact regarding the debt. But not a girlfriend.
Contact with third parties is restricted only to attempts to obtain location/contact information of a consumer, and such contacts may not disclose that the consumer is alleged to owe any debt.