If you can do magick tricks as proof, then you have a much better chance. If you can spin a psi-wheel or make a cloud rain in front of her, then just ask her to cast the demon out of you in the name of her god. When nothing happens, and you are still able to perform "supernatural" actions, it will be a very strong case against her religion.
That would be unwise, as it takes quite a bit of spiritual power and energy to make "displays" of magick, which would be dismissed as pure "evil" witchcraft by a christard, and if Sero was that advanced, he could simply begin speaking to his mother's light body and begin influencing her dreams and subconscious thoughts.
According to the bible, humans can't perform magick. A demon must be helping them. It also claims that god gives all christians power to cast out demons. However, the Salem witch "trials" show what a little paranoia can do, regardless of what the bible says. That's why I warned Sero to be very cautious. Although I don't know his mother personally, you are probably correct. I suspect she can't be converted in this life.
A psi-wheel is actually really easy to spin because the friction is so low. Have you ever tried it? Make sure you try both directions to rule out the influence of air movement.