There are lots of children in informal "foster" situations like this.
At 16, she can leave home but officially needs parental permission to do so. (In reality this is rarely enforced.)
If you can get an "official" placement endorsed by Social Services, you may be able to claim CB, but don't count on it. You won't be eligible to claim maintenance etc. So economically you'll need to consider if you can meet her needs in terms of food and clothing, school equipment, allowance etc.
In terms of housing, while initially they could share a room, seperate bedrooms would be best to avoid straining the friendship.
As this young girl clearly has suffered a lack of emotional support in her birth family, she may "imprint" on you as a mother figure and be very clingy, at least initially. It will be important to set expectations for this with your DD and maintain some ground rules about DD having time with you 1 on 1 so she doesn't feel pushed out.
You need to commit to spending time and energy supporting this girl to seek mental health support, initially probably through CAMHS. (I know CAMHS have a bad rep in some areas but my own experiences with them have been largely positive.) You need to be able to faciliate her to get to appointments, attend ocassional catch up sessions, agree contingency plans, arrange contact sessions with her birth parents if she wants them, give her support to reach her academic goals.
You'll also need your Dh/DP on the same page as you. They can't expect that things will just carry on the same but with a +1. They need to state what input they will have and how they plan to help this young person to fit into your existing family unit. For example, assuming they do a solo activity with your DD, will they be able to include DD's friend in this activity, or can they commit a similar amount of time to 1 on 1 activities with friend?
Good luck OP, please PM me if I can help further.