Taking on someone else's child good/bad idea

My youngest daughter has had a best friend for quite a few years they talked all the time, me and my daughter talked all the time too so when her best friend was having a hard time at home she talked to me for advice. I went through a rough childhood having parents who didn't care , who thought it was funny to see there kids suffer or in pain so I knew how she must be feeling, Me and her friend used to talk and I tried to help but you can't say to much you can just point them in the right directions and tell them who they should talk to.

This went on for quite some time I would hear them talking with them both getting upset, her parents were super strict, she wasn't allowed to take her phone to school, her parents were allowed to read her messages, she was controlled even when she got a apprenticeship they took most of her money, they had her doing all the housework and looking after her younger brother whilst they slept all day,

I have 2 daughters my oldest has ADHD, ODD & Aspergers so bringing her up was really hard, We did it though and were super proud of her, she has moved out and is doing really well, when she has her ADHD/Asperger moments mum is always there . She moved out so we sort of thought yes our lives will get easier, took over a year to empty her bedroom, its a box room , before she left she couldn't get into it to sleep so slept on the sofa, she was a hoarder and saved everything. Took a while but we managed to get it empty, we had plans for that room it was going to be a dressing room for my youngest, somewhere to keep all her make up and hair stuff.

THEN

I got a phone call from My daughters friend saying she had walked out, she couldn't take anymore. She was just 16 out on a dark night, her parents had opened the door for her then closed it after her. So I told her to come here, that was 2 years ago come October, she's been here ever since We decorated her room, bought a sofa bed and new curtains and she made it her new home. She is a lot happier, she has cut her parents out of her life for the time being (they haven't tried to get her back) she is now apart of my family and has fitted in after a bad start.

I asked her if she was interested in getting a place of her own she said " I'm never leaving here ". I think it was a bad idea at first but now happy that we have done it.