I remember an episode where a "family" (all actors) was out at Sunday dinner, and they were all dressed in the traditional regalia of a religious cult (women in plain dresses, long hair up in bonnets). They had a conversation wherein it became obvious they were marrying off a teenage girl (who couldn't have been a day over 15) to a man who was comfortably in his seventies. They were telling the girl she would learn to submit, it was God's will, yada yada yada.
A few people stepped in, called the cops, etc.
Here's the thing: What possible outcomes are going to manifest by stepping in, calling the cops, whatever?
First of all, in areas where such cults are known to exist (such as the towns where the FLDS/Warren Jeffs cult were active), cops have been known to look the other way at these crimes or, even worse, interfere when state-appointed social workers came a-calling.
Second, what "help" is a stranger (or anyone, for that matter) going to give that girl? Move her away from the only family she's ever known, and into a foster home? Then what? She's going to turn 18 and have little education and no job skills, and she's going to be thrust into a world for which she isn't equipped.
Stopping religious cults from human trafficking of underage girls isn't a thing that's going to be accomplished by random strangers at a restaurant. It's going to require a massive, top-down effort, starting with the Arizona or Utah or whatever state government, down to its social-services infrastructure, to law enforcement, and so on.