Your friend's daughter is physically inconvenienced by it, boys in her area tease her for it, and is not allowed to cut it even though it's been 8 years since the offending incident that prompted the "punishment".

It's clearly too much for her and frankly, what she does with her hair should be her choice.

On that note, your friend really seems like an abusive POS and is using her daughter's hair to keep her in line.

Since the daughter is 18, and Child Services likely won't do anything, you should really consider finding a way to let her know she has options beyond staying and putting up with everything.

Assuming you're in the US, here's a link on how to do this:

womenshealth.gov/relationships-and-safety/get-help/how-help-friend

but I'd also encourage you to find more specific advice to whichever country you live in, if you can.

Regardless of what you choose to do next, posting here is not going to clear your conscience as much as helping however you can.