Don't mind Richard. He's not a hypnotist and has no experience with hypnosis whatsoever.
Can they undo this via self-hypnosis? Can a second person, perhaps a professional, undo that neural pathway or however it is the trigger word works?
The short answer is "Yes". Depending on the situation it may be more complicated or it may be less.
whether it's because she so believes it's true or is anxious enough about it or anything, it's worked on her, with or without her consent.
That is sorta true, as hypnosis is intimately tied with expectation. That is not to invalidate her claims, of course. Her experience is very real, very compelling, and simply saying "it's not real" will do nothing to break the effect because it will just prove to her that you don't know what you're talking about.
One of the common "party tricks" people do with hypnosis is to "stick" people's hands to the table. What is actually happening isn't that they "can't move their hand" but that they can't *intend* to move their hand because the hypnotist has done such a good job of crafting a scenario where the subjects attention is locked onto the idea that it cannot work. This often takes a bit of active work to sustain, since the moment attention drifts from that idea and starts to encompass the idea "maybe I *can* move my hand", the intention can be formed and that's all it takes.
Kev Sheldrake (one of the guys who used to post here) talked about how he'd like to mess with his friends' attempts to stick people's hands by reminding the hypnotic subjects "you can stop imagining whenever you want". While this may seem very similar to Richards "hypnosis can't do that", it is very different in an important way. Kev is lending *permission* to stop thinking a certain way instead of acting to invalidate it. Kev takes for granted that one *can* have their hand stuck to the table, so long as one's mind stays in that "imagination".
Does anyone have any idea what to do, anyway? Is it possible even for her to get some placebo effect of undoing it, if a placebo effect is all this is? Obviously I'm not taking one person's word for it never being possible just like I'm not immediately believing that it absolutely must be, but I've heard her experience, it is influencing her, and it is terrifying to her, and so I want to fix it regardless.
Yes. That's a very good mindset to have both in general and with undoing the damage of shitty hypnotists in particular. Bad hypnosis comes from focus on a bad idea to the exclusion of better ones, and the main reason this can persist without the hypnotist there to constantly reinforce the suggestions is that fear can do the reinforcing itself. If you get stuck in the "omg, what if I never recover!?" stage, then you never get to the stage of realizing that you can stop "imagining" whenever you want.
This goes for both "you" as the person who has been hypnotized or "you" as the friend. As an outsider wanting to help, your job is to open up the span of her attention to encompass the idea that she can just not respond to the suggestions, because that *is* the process of "rewiring" her brain to no longer responding to the suggestions. If you were to fall into insufficient skepticism and accept her fears as "true and unquestionable", then you'd essentially have fallen under the spell yourself and would just be reinforcing the troublesome (and *false*) idea that she can't stop responding to the suggestion. If you were to fall into insufficient empathy and fail to see how her experience can be a compelling reality to be dealt with, then you'd essentially be shutting yourself out of being able to reach her. So far, you seem to be doing this exactly right, which is commendable.
Having a good friend in a time like this can be invaluable, especially since the source of answers on how to deal with this problem ("random hypnotists on the internet") is uncomfortably close to the source of the problem. I've been talking to someone who found herself in a similar situation, and by far the most difficult part of helping her is that she has learned a pretty reasonable fear of actually engaging with things intelligent hypnotists say.
Without having talked to her or knowing more about the situation, it's hard to give specific advice about what would be the best way to proceed. A competent hypnotist could certainly help her, but professional hypnotists can be kinda hit or miss and cost money either way. It also might not be necessary. One thing to look into is the "deep reset" type files that some hypnotists have out there. They're free and you can read reviews/screen them yourself, so there's really not much cost to trying that approach.