I think cult deprogrammers usually operate on people who have been removed from the cult's influence. I don't know how effective they would be on a person who hasn't left the cult yet.
People join cults due to some emotional issue in their life, not for rational reasons. The cult fills the void so to speak. To deconvert someone requires that person reopen that void and it can be very painful so it isn't likely to be all that successful.
JWs, as an example are a group I attempted to reason with once and it was a failure. Basically they came over and wanted to tell me their spiel. I agreed they could return the following week and I would listen to anything they had to say, but in exchange they had to read a short paper I would write for them.
So, I wrote a paper that cited watchtower publications, a former Watchtower VP and other inside sources as well as court cases against the organization showing the false prophecies and abuses of the organization.
They accepted my paper, put it in their briefcase and proceeded to give me their spiel. They assured me they would read it later. The next week they returned and I asked if they had read it. They hadn't.
Another time I worked with a JW and I verbally gave him some of the information. He dismissed all of it as the words of an apostate.
So, even flat out, undeniable, easily verifiable facts are worthless if the individual has fear (or other strong emotion) based reasons for being in the cult to begin with. Cults use the human brain's natural self defense mechanisms against the member.
Good luck finding something that works.