All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We have already clarified in Fataawa 82393, 85126 and 85383 that it is forbidden to visit graves and ask the dead to fulfil one's need, as this is a major Shirk [associating partners to Allaah].
Therefore, you should not accompany your mother when she goes to those places, because accompanying her is helping her and confirming her falsehood. Rather, you are obliged to endeavour to prevent her from going to those places, with good admonition and soft words. You should clarify to her that what she does is at least an innovation if not Shirk.
For more benefit, please refer to Fatwa 86582 about the innovation of reciting the Quran at graves, and Fatwa 82339 on the meaning of a Wali [the one who loves Allaah and whom Allaah loves].
Allaah Knows best.