"find out what's missing about your child's life that would convince him to leave it in the first place"?
It isn't meaningful to blame a parent for Dave McKay.
No one deserves someone in their life like McKay in my opinion.
Some people just have the bad luck to run into someone like McKay.
It seems to me that McKay is something like a con man and poor Joe Johnson was a teenager and simply not equipped to deal with him.
Groups called "cults" may seem like "a fresh new perspective", but in fact they are really a rather an old con game, i.e. usuallycontrived by a sociopath who exploits people.
The isolation you mention is a necessary tool to cut people off from any outside frame of reference and/or accurate feedback. It's a control technique.
This Encyclopedia of Sociology excerpt explains the process of coercive persuasion often used by leaders of groups called "cults".
This is an explanation of "coping strategies", which can be useful when dealing with cult members.
I agree that families shouldn't be needlessly confrontational, but that doesn't mean they should simply give up and let the group leader exploit someone without developing a strategy to hopefully respond and help the person involved regain their independence and leave the group.
This is a collection of articles concerning recovery from groups called "cults". There are some common issues that most ex-members struggle with after leaving a high demand group.