For a long while their innermost secrets were just that, secret. Members are tight-lipped on any real elaboration, and won't engage with outsiders.
It also makes it really difficult for outsiders to present any countering logic to their beliefs. People aren't gonna tell you you've fallen in with crazies, if you don't tell em the dumb shit you're being told is real.
But a lot of it is now out in the open, if you know to look. Even then, the church AGGRESSIVELY wields the law in an active attempt to suppress public knowledge as much as possible. But as even top-level members have left the cult over the decades, even the innermost bullshit has been exposed.
It's probable that your daughters boyfriend himself knows much less than what is available online, because members are discouraged from interacting with any "harmful" media so as to not grow disillusioned, and aren't told anything substantial until it's way too late to painlessly pull out.
It's all quite deliberately set up to be as insidiously prolific as possible, while minimizing the chances people will leave.
And if people do show start to show signs of wanting out, the gloves come off really quick with stuff like blackmail and legal action.
Even its tax exempt status in the US is a complete farce, yet it lends the cult an air of legitimacy.
Allowing a person to think for themselves is always the best option.
But allowing her to accept scientology should not be an option, and isn't what I was suggesting.
I simply assume that OP, looking into scientology with a critical eye, is likely to have a good relationship with their daughter. And that the daughter being of dating age, and the offspring of someone seemingly reasonable, they are both capable of having an adult conversation. One that won't end with their daughter going "fuck you, I'm cutting you out of my life forever" but rather with their daughter accepting reality and intergrating the facts into her mind in a way where they can't be easily subverted.
If that isn't the case, then the heavy handed approach is absolutely warranted.