My experience is that after some time and level of proficiency in a language, some just stop learning and caring. I think that is a pity because I believe that with revisiting the rules in interesting exercises one can fix the mistakes even later.
It may help your Aunt to learn German (again) with beginners books. There are good self-learner books with interesting exercises. They are less than 20€ per level and if you are already good, it should be easy to do multiple pages per hour.
It would be important for a native speaker to check the answers, though. And also the solutions should be crosschecked with the books solutions, if there are any, because many Germans don't know all the rules correctly. If in doubt, just stick with the book.
My aunt, an American who has lived in German-speaking countries for 10+ years and can say almost everything she'd want to tells me she still gets treated less seriously because of subtle mistakes like that in her speech.