In my opinion this is a serious matter, however good your intention may be you should put your safety first
Unfortunately we don't live in a very safe world and it depends a lot on where you are, how your acquaintance will react to this in a positive way or not, he may be open to it but if he is under Christian influence or any other he may have a negative reaction to it.
I've read an article by Maxine about how she managed to help someone towards the end of her life (age), but it's Maxine I'm talking about, she knows what she was doing.
I speak from experience, as much as you might want certain things to bring someone closer to Satanism, it can be an unpleasant experience.
As I can tell you, we're dealing with the soul, magic etc, things can seem good at first but then suddenly take an unexpected turn and it can end up on a very serious path.
As something I'd like to say, I'd say be careful, Jews also frequent this site (unfortunately) and for them to look at this situation and start rubbing their disgusting hands seeing you ask for advice is not something hard to imagine, what I mean is that in addition to advice like you're asking for think for yourself as well, unfortunately this world is not that safe.
You say your relative is a medium, but do you know if he's a Christian, for example? What does he practice? In any case, I think you should be careful with this kind of thing.
I think that to engage in this kind of thing I myself would prefer to create solid foundations with magic, financial security etc, so that if any problems arise you can defend yourself.