Hi Heather, welcome to Permies!
Most of my experience in the matter of internships, WWOOFers, etc. is indirect, as we have never had interns or similar on our ranch. So others may be able to give you more pertinent information, but I'll start things off.
Most of your formal internships or opportunities to formally spend time and learn on farm (or maybe even paid internships, which to my knowledge are extremely rare, if they exist at all) are going to be on larger, more well-known farms--what I think of as sort of "celebrity farms." Salatin's, Paul Wheaton's, Geoff Lawton's etc. Keep in mind that most of these are not paid, to my knowledge. If your daughter wants to get paid, she'll probably need to get hired for a specific position (perhaps animal care, cooking, cleaning, etc.). Or she may possibly be able to arrange payment for specific tasks that the farmer/host wants done above and beyond what is done by most interns, if she proves to be a particularly adept and/or hard worker, although I wouldn't count on this at all.
WOOFing and many farming "internships" are indeed "dirt cheap manual labor gigs" of a sort (although I don't wholly agree with the negative tone of that statement). The idea is that the intern exchanges their labor for the opportunity to learn, and possibly for some basic sustenance, not for money. And yeah, often this involves a degree of sleeping rough, etc. (many people who participate in this sort of thing are into backpacking, etc. and are comfortable with this; if your daughter is not, she should look for something that provides shelter and/or meals, or maybe view it as a chance to try out less luxurious living, especially if she is eventually planning to live off grid or in similar arrangments--many of us find that we like it!)
Here's the thing--most farms do not have an excess of money. If they are going to spend that money to hire someone, they're not going to spend it on someone young and inexperienced who's going to need a lot of guidance and supervision. They're going to hire a professional to complete a task that is specialized or difficult and needs doing. Honestly, the labor provided by many (most?) interns does not allow the farmer/rancher to even break even in terms of time spent supervising them, dealing with interpersonal conflicts and complaints (if there is more than one intern), arranging food/housing/transportation, not to mention teaching them...most interns are inefficient (which is natural and to be expected--no beginner is efficient). And often, if not given a lot of guidance, they will just sort of drift and shuffle about and become a liability in terms of time, work, and efficiency.
What I would be looking at in your/your daughter's shoes is whether or not she will *actually learn* some valuable skills, not whether she will get paid. If all she does the whole time is dig holes or move rocks or pull weeds or something, then yeah, she's not going to get much value out of that experience (although some of that is par for the course). If she is not very experienced in specific farm skills and is not proactive about it, she may very well end up as the general gopher around the farm because the owners are busy and it is easier to assign someone a manual task that they can't screw up and take care of the more delicate things oneself.
It would probably be best if she had some specific goals--does she want to learn how to operate heavy equipment? Set up PV systems? Run a CSA? Plant a forest garden? Restore a watershed? Grow vegetables organically? Manage a holistic orchard? Learn about medicinal herbs, timber framing, wildcrafting, primitive hunting skills?
Compost with worms, breed chickens or plants, save seeds, milk goats, build fences, advertise and market effectively, etc? And then try to find opportunities that will allow her to fulfill those goals in particular, and discuss this with the hosts beforehand and judge their general enthusiasm for passing on these skills (and how much they actually seem to know about what they're teaching).
I would ask if the teacher/farmer is still in contact with previous interns and what those relationships are like, what those students are doing now, and if they would be willing to pass along my contact info (or vice versa) so that I could speak with the former interns about their experiences on the farm.
I would also ask how long they have been farming, and their goals for the farm in the future.
I would also try to get a general idea of their general approach--are they more oriented toward developing a market and making a profit in an ethical way, or are they more interested in self-sufficiency and only interested in profit insofar as it keeps their land from being repossessed; are they particularly interested in advancing knowledge about and sharing a particular skill or approach, but don't do much else, or are they generalists with a multitude of skills but no special area of expertise; are they "plant people," "animal people," or "infrastructure people," etc.
Just have a conversation with them and see if they feel like a good fit for your daughter's goals and general comfort levels and life philosophy. If they rub her the wrong way (or vice versa), flee.
If you/she are really dead set on a paid internship, my gut feeling is that you will be disappointed, but there are opportunities out there to hire on for a specific position, as I mentioned. Problem is, she'll be busy doing that and won't have as much time to learn. Depends on her priorities.
Honestly, a lot of farm experience is more about building confidence than it is learning specific skills. You can pick up the skills. Interning on a farm will give her access to things that would otherwise be difficult or prohibitively expensive to access (heavy equipment, large animals, woodlands to manage, etc.) Getting a sense of how/when to apply the skills that she learns to best effect comes with experience and can be greatly accelerated (and a lot of pain avoided) with good mentorship. If she can make connections with people who will support her into the future, that will be way more valuable than any technical knowledge she picks up. Frankly anyone can figure out how to grow vegetables or raise goats or install a solar array or whatever with a little self-directed learning and trial and error; she doesn't even need an internship for that.
If she wants to do primitive survival stuff involving plant identification, tracking, etc. (uncommon skill sets with the potential to be very dangerous if messed up--somebody eats the wrong mushroom, etc.) finding a good mentor would really be to her advantage, and most run of the mill farm internships will probably not advance her very far towards her goals.
It sounds to me like her biggest obstacles in the future are going to be saving enough money to buy sufficient land and developing enough of a skill set, reputation, and network to find enough students to make payments on that land and cover her expenses (especially in the beginning, before the land can provide most of her needs). I would give serious thought to how each and every move she makes gets her closer to overcoming those obstacles. What I would hate to see is if she just ends up as a serial intern/WWOOFer and it never really gets her any closer to getting her own place.