Is it sufficient for my grandmother to recite only the surahs and verses she has mastered, or should she continue to recite the rest of the surahs despite the difficulty in pronunciation, and is she sinful for not being able to recite correctly?

Reciting the Quran is a great act of worship, but it is not obligatory. If the grandmother wishes to recite, she should limit herself to the surahs she has mastered. If she is capable of learning and correcting her errors, then she should refrain from reciting with errors until she learns. However, if she is unable to learn and her errors are few, she may recite. But if most of her recitation is erroneous, she should desist from reciting.