Ok maybe I'm missing something here, but I can't get this thought out of my head after reading the OP.

Your GF was hesitant to get a CC because she was worried she wouldn't be able to control her spending.  Now she has a CC and refuses to use it for the same reason.

Isn't refusing to use it a sign that she can control her spending?  In my mind, keeping a CC in the sock drawer is equivalent to managing your spending from a practical point of view.  Does that make sense?

If you can keep yourself from using a card, why can't you also keep yourself from overcharging?

Seems like she's worried about nothing!