With witnesses someone has to be mentally ready to accept the possibility that they have been misled. Telling them too much too soon just forces them back in and plays to all their fears. It is also true that for some the cult fufils a need since it is hard to see how they would fit in or have any social life without the meeting structure which also insists that other people put themselves out for them. For parents there is also the issue that accepting that they have been misled means they also have to accept that they brought their children up in a harmful cult and did not give them a normal or happy life. Steve Hassan is right that if they are happy in the cult there will be little motivation to change.
Trying to get someone out is a balancing act weighing up what the consequences will be for them with the benefits of mental freedom.
Approaching the subject obliquely has the best chance since any direct challenge will cause them to retreat into the cult and raise a barrier to any further discussion.