Great subject. They do indeed advertise themselves as a mediator or go-between.
Also another thing is cults are often putting themselves as mediator. The moonies did this as well. This might help someone to see that this is a very common thing. Maybe you could show them the difference between what the bible says and what a group is saying about who or what is supposed to be the mediator. Admittedly I haven't done much research on the subject but there seems to be a direct connection between dispensing blood and being the mediator e.g. Moses sprinkled the blood of the sacrifice on the people . Also prayers going through who? There is probably more than this. If you really knew what you were talking about I think you could convince them by "Reasoning from the Scriptures" .