I know that you probably have enough responses to your question, but I would like to give you one more. I have read through the responses that you have already received and as you have found out the advice is great.
A little about me, I am 30+ years old and have long hair past my waist.(and have had since I was little). Occasionally I will sit on it or close it in the car door. I am always being asked how I keep it looking so nice. Hair dressers usually ask how I keep it so healthy. If your daughter would like to keep her hair long, I thought you might like to know what I tell people.
First and foremost, NEVER brush wet hair, ALWAYS use a wide tooth comb(wide tooth picks work well) when dealing with wet hair. Brushing wet hair will cause more split ends by pulling on knots and causing the hair to break. When combing/brushing, start from the bottom and move up. This way you don't bring all the knots to one spot, you work them out as you go. Also, hold the hair by the hair and not the roots. This gives you a bit of play when you come to a knot, allowing you time to work it out before you pull.
Conditioner is a life (& time saver). If you use a shampoo and conditioner in one and it doesn't seem to be doing the trick, use some extra conditioner.(Don't go overboard though as it will build up on hair.) Another thing, when rinsing hair for the last time, use cooler water. It doesn't have to be cold, just cooler than your normal shower water. When you rinse with warmer water it causes the scalp to produce more oil which in turn will cause the hair to get dirtier faster. I have never knowingly tried the detanglers, but I think mom did. I guess they worked, I still have long hair.
Also, switch brands of shampoo and conditioner from time to time. All brands of shampoo and conditioner will build up on your hair over time and when you change brands you will remove most of that build up. I use Suave most of the time, switch to another brand for a bottle and then go back to Suave for a while. I can really tell the difference when I switch brands.
As you have figured out, braid her hair before she goes to bed.(I do it every night- takes me about 3 minutes.) This not only helps with the knots and tangles in the morning but will allow a better nights sleep. When you have long hair you tend to lay on it and when you try to turn over your stuck. Then you have to wake up enough to move your hair out of your way. A braid makes this much less likely. Another thing about a braid, if your daughters hair is very thick you may not want to braid it when it is wet. If the braid is really thick, the hair may not dry all the way and become sour. You can still get the nice waves by braiding her hair, just leave the braid in a bit longer. Maybe until after breakfast.
The last piece of advice I can give is to have the split ends trimmed every so often. The split ends are usually what get tangled the most. You can trim most of them by trimming about half an inch. This will make a HUGE difference. I know that when my hair starts to get extra knots, I need to get it trimmed.
Sorry this is so long. (and late) I hope it helps.