Almost any person can fall under the influence of sectarians. There are many known cases when the most sane and realistic people became weak-willed slaves of some "guru". They renounced their former life, gave all their property to the sect, and any attempts to reason with them were met with open hostility. In those rare cases, when a person realized what kind of swamp he was dragged into, it was very difficult to leave the sect: psychological pressure and even physical violence were used.
Contact a specialist, because a lot depends on close people, relatives of the newly-minted sectarian. They must clearly understand that this is not about licentiousness, whims. A person is simply well "brainwashed" and very effective, depriving him of the ability to critically perceive the surrounding reality. After all, the technique of attracting new members in sects is set at a very high level. Therefore, one cannot do without the qualified help of a psychologist, and often a psychiatrist, just like without the support of loved ones.
But what if a person stubbornly does not consider himself sick, in need of help? And so it happens in the overwhelming majority of cases. This is not an easy question, because according to the law, it is impossible to forcibly treat. Here you need to get the advice of a qualified lawyer. There are circumstances when, by a court decision, it is possible to obtain a compulsory psychiatric examination of a person.
At every opportunity, stop the communication of a person close to you with the sect and the people who are in it. Persuade him to leave for another place, leaving no contacts through which the cultists could find him. If he does not agree, limit his communication with the outside world yourself. Just try not to overdo it, because the article of the Criminal Code on the forced restraint of freedom has not yet been canceled.
Sometimes communication with religious leaders helps. Try to seek help from a reputable, respected priest (mullah, rabbi). There are cases when sectarians, after conversations with them, seemed to see their sight.
If you have good reason to believe that a person is being held by a sect forcibly or with the help of threats, contact the police. Be persistent if someone tries to dismiss your statement.