If your girlfriend is not going to take the adult exam, you should persuade her: Why torture yourself with grades? What if it fails? Everyone is not born to learn mathematics (the above answers are all very good, so I won't go into details.

If you still want to take the exam: don't discourage her from learning mathematics too much, but tell her the key to learning: focus on the foundation, ask more questions, make a good book of wrong questions, and classify according to methods. If you really study hard for a month or two, you will gradually understand (mainly accept the way of thinking in mathematics) and your study will gradually get better.