1) Get this brush:
It is well worth the price. It works. My daughter has it.
2) Use a leave in hair serum or conditioner. Garnier Fructis, is the brand I like. Good stuff.
3) Use a satin pillow case for her
4) TRIM her hair REGULARLY. Because, dry ends and split-ends, makes the hair, tangle a LOT more.
5) Do not shampoo her hair everyday. That will dry it out, and this makes it tangle more.
6) After shampooing, do NOT rub her hair with a towel. Blot it.... and dry it. Rubbing the hair roughly while wet, makes the hair worse and tangle more.
7) If I were you, I would cut her hair. To a more manageable length. Tell her... that some girls, get all kinds of gross gunk on their hair being that long. And when sitting down, it does touch the seat etc. And some kids get head lice.