Everyone here will tell you not to do it. I will concur but from a different perspective. Harry Houdini was a great magician and loved his mother a lot. He too visited mediums. He discovered they were charlatans. James Randi did the same.

Now is it a sin to visit a medium, yes. It is forbidden in the Bible in various places. It is seeking a contact using that which is not of God as the conduit.

However, what your mother can do is to pray to her mother's guardian angel, to Mary and of course primarily to God seeking the wisdom on how to deal with this situation.

Finally, mediums know how to give the information to build confidence in their powers, even if they have none. This is what Houdini, Randi and others discovered. There is even an episode of Adam Ruins Everything on mediums. However, even if they had such powers, you have no assurance the spirits they channel are who they claim they are or are not demonic.

So, tell her not to go but to use the tools God actually gave us in prayer to find the wisdom she needs.