Try to make her understand through logic. I don't know if you've tried it but I'll share my thoughts nonetheless. Assuming that the Qu'ran and hadiths have changed, we can easily conclude that Islam is not the correct religion because it is then obviously clear that the text is not from God. This conflicts with the fact that God is supposed to guide us. It's his duty, if you will, to make sure that the truth is there even though it might be hard to find. Ask your mother why she believes Islam is correct? Is she just blindly following it? What's her reasoning because if Qu'ran and hadiths are actually changed you can be pretty sure that Islam is the wrong religion. Where does she believes guidance comes from?