Let me try to summarize to see if I understand your question. Your friend has some issues. She wants to get some help, but doesn’t feel she can afford therapy. Her parents are offering to pay for her to attend Co$ classes. (Are these a seminar of some sort, or standard auditing? Does it matter?)
You are concerned because you think Scientology is a waste of time and potentially harmful. You don’t want her to be hurt or used and manipulated or otherwise emotionally injured. You are not sure if the attending the classes will just demonstrate how useless they are to her, or will get her wrapped up in the nonsense. Yet you are afraid to come on too strong with reaction, perhaps because of her parents’ involvement?
You are looking for advice on what to tell her, and whether to support her taking the classes in hopes she’ll get over it or get bored with it, or whether you should try to talk her out of it, and if so how.
As far as the classes just boring her or being worthless and driving her away from it, I don’t think you can count on it. While it may have this effect on some people, those are typically well-grounded skeptical types. Someone having emotional issues is vulnerable to anyone attempting to help them, even if the help really isn’t help. Also, these people are good at convincing people they need this. If they weren’t, they wouldn’t be around. Remember Co$ is a money-making venture. Like any money-making venture, sales is key. They have good sales people. My advice is that you should do something to show you don’t think it’s a good idea. After all, you don’t think it’s helpful to her, so express that to her.
As for what to say to convince her, how strong to react? Um, I’m not sure here. If you can provide even some of the material above in printed form so she can read it, that might help. Just have a serious conversation with her expressing why you think Scientology isn’t what it claims to be, and why you think she’d be better off skipping it.
For points to consider…
Co$ is founded on a lie. It is a completely fictional fabrication purely for making money. That is per the founder himself, L. Ron Hubbard. Of course other religions have just as crazy and unsupported notions and mythology.
They actively strive to suck you in and get you to pay for ever more levels of attainment - the rise in “spirituality” is purely a function of how much you spend.
They are a cult. They manipulate your thinking, offering mental abuse in the name of curing you.
They are scary. They do not appreciate critics, and are as dangerous as the Mob in protecting themselves. Including harassment, legal suits, threats, intimidation, breaking into offices and stealing documents.
They claim to be science, but are the biggest load of crap completely unsupported by science. They are pseudoscience. The engram crap and stuff is discussed on the links provided. E-meters are bogus. It’s all just window dressing so you’ll cough up money.
How much of this you are able and comfortable sharing with your friend is a judgment you will have to make.
I hope this was some help.