I lost a LOT of hair last year- turned out to be a mix of stress and seborrheic dermatitis. Annoying thing is that it's very mild dermatitis, nothing like the horrible pictures on the internet, and still my hair was coming out all the time...v. annoying! I lost about 2/3 of my ponytail width, but luckily I had very thick hair so it doesn't look too terrible. It still starts to come out more if I don't shampoo with nasty chemical shampoos every other day, which is another annoying thing because I prefer only to use mild stuff. But anyway, if her head is itching or she's getting stressed, it could have provoked dermatitis. Or- but this is quite rare- her scalp could have had an allergic reaction to shampoo, which would manifest itself like dermatitis in that you'd lose hair, except no extra sebum.
If she's had a very sharp shock around three months before it started, she could have telogen effluvium, lots of info on the internet about it. If she has an underactive thyroid or polycystic ovary syndrome, that could be a reason, or just because it's Spring- some unlucky people shed heavily after winter. Maybe her iron levels are too low.
Don't pay for bottles of Biotin- they're unlikely to do much unless she has a biotin defficiency, which is very unlikely. Alopecia is another possibility, but it's doubtful. Does she tie up her hair a lot? When I was younger I couldn't stand having hair around my face so I tied it up tightly every day and in photos I can see my hairline went back a couple of cm- it all grew back. In any case, since she's young, it'll almost definitely be nonscarring. Something heartening might be to check if the fallen hair has bulbs/roots on the end- if they do, it means a new hair has pushed it out and is growing.
She might want to avoid 'minituarisation' of the follicles by using some sort of clarifying shampoo, or cutting out dairy or whatever works for you. I have tons of hairs which are thinner at the ends and thicker at the roots, because it minituarised and then sorted itself out. But if it doesn't sort itself out, you can just keep regrowing fine unpigmented baby hair. If she becomes desperate, have a go at oil pulling.
She might be tempted to start not washing her hair as often or not brushing it because of all the hair that comes out- do encourage her to, because it's better to have a little shock everyday than an awful one once a week when you do do it, because it collects.
I'm sorry she's losing her hair, it does feel awful! The thing is, it's the old thing of being self concious. It looks better to everyone else. The important thing is to keep her confidence up about it, if she's just washed it, pay her a compliment about it looking full and healthy, ask her about it from time to time. Hope it stops very soon!