I would try and find alternative accom close by.. Help her out as much as you can as a friend and offer her a shoulder to cry on or find her someone she can confide in...

Personally I wouldn't be taking on another teenage girl if you are just getting on your feet with your DD. Also the other thing that would concern me and I am not being judgemental but if the mum is doing drugs then I would be worried your DD and her friend might get mixed up in that.

Plus you have 2 younger children to consider. so yeh in my opinion I would be helping without taking the girl in..