First question: what does she shampoo with?
Yes, definitely braid it at night. My mom always did mine as child. She also needs regular trims-wait no longer than 8-10 weeks. Split ends will more easily tangle than trimmed hair. As a hairstylist, I wouldn’t recommend Mane n’ Tail as it is very heavy on the hair, caoting the hair (hence the slippery floor!) and has too much protein for human hair and can cause breakage long-term as a result. I dealt with many negative consequences of that stuff when I was working.
DO not brush wet hair, either. Wet hair is more sensitive than dry. It stretches and break and frays when brushed when wet. Just use a wide-tooth comb when combing out and braiding.
A spray leave-in is a good idea, however. Vinegar alone isn’t enough to keep the hair conditioned at that length. Her ends are old hair, to put it bluntly and it needs conditioning. ONce a week or every two a deep conditioner would be worthwhile. A natural solution is fine. Coconut oil is a great choice.
Since she has a lot of hiar, it’s possible that she isn’t rinsing all the shampoo out well enough. Hair with build-up will get tangled easily, matted actually, and be difficult to comb through.
If I think of anything else, I’ll chime in.