im going to tell her about the B vitamins. poor girl is so pale and tired all the time...and shes also worried people will think shes faking it for attention, because apparently there are lots of videos of young girls doing this, but its not like shes doing anything of the sort. i used to give her these giant salt tablets but they made her stomach hurt. i feel bad for both of you, having to deal with this issue, because it definitely has to suck!!!
Has she had her iron levels checked? Anaemia in young women is very common and can lead to similar symptoms.
That's the first signs of anemia. If she has heavy periods she may be loosing lots of iron.
Iron uptake is also reliant on having enough vitamins to process it into blood. I'd send her to a doc to get blood work done. Iron and iron reserves used to be like 15USD for a check.
I don't think many people know this, but while mild iron anemia is common for women of child-bearing age, iron is not the ONLY deficiency that causes anemia. B vitamins and others can cause anemia as well.
Anemia is very common with POTS. It's fairly normal that one when function of autonomic/dysautonomic nervous system has regulation issues, other sections may as well, or may develop issues later. Those systems work together to control basically every unconcious action your body does - not just maintaining blood pressure and volume, but breathing, heart pumping, digestion, bladder, etc. A lot of people with POTS also deal with malabsoprtion, heartburn, breathing issues, heart palpitations, slight incontinence, etc. For me, even though I practically eat like a carnivore I still have relatively low levels of iron.
I really can't recommend enough that your daughter drink electrolytes religiously, and take supplements - a good pre-natal supplement is fantastic with POTS, as well as a B complex and an iron supplement. I think they make (or at least market) supplements and electrolyte mixes specifically for POTS but I haven't tried any of them, the above combination works for me. Electrolytes in the most basic form are basically saltwater, but in a format that's easy to digest and should avoid any issues she has with stomach upset that the pill could cause. I would also recommend she looks into compression garments, pants like fighter pilots wear. I invested in a Polar H10 to keep track of my heart rate and it's helped a lot in tracking potential episodes early, it's about $100. And then there's your basic lifestyle changes - stress and heat are common triggers, some foods do it for some people too. Try to eat a balanced diet, exorcise appropriately, sleep well, etc. If she's in danger of random bouts of fainting while in public, she could potentially get a medical dog to alert her to episodes and catch her if she falls, but those are very expensive (covered by most insurance though). Physical therapy is really important if she can afford to get that - the whole "go until I drop" thing was my first instinct but in the longrun it apparently fucked my exorcise tolerance. The key is being active every day, but not overexerting yourself, and eventually you can start actually raising your endurance and do more without triggering symptoms. It's aggravatingly slow, though.
I will say that dealing with POTS is really upsetting, even aside from the physical limitations it places in you. I struggled a lot with accepting I had a problem that I couldn't just continue to ignore. I also feel like I'm faking it somehow, I'm always afraid people might think I'm faking it for attention and now I rarely leave the house. My husband can usually tell before I can when I'm having an episode and I get ghostly pale, so I know it should be obvious to an extent, but I hate drawing that kind of attention to myself. It's also embarrassing to admit in casual conversation that I can't work or drive when I look perfectly healthy right then. Mentally it can screw with you a lot; I can tell by the way you talk about your daughter that you truly care about her, and I'm certain that means a LOT to her.