Why was your daughter in rehab? Is she using drugs or alcohol? Suicide attempt? I would talk to the therapist about all of this, meaning your concerns, her sexual identity, and the fact she is dating someone she met in rehab. As others have pointed out, you're not supposed to get involved with anyone in rehab, and that's what she is doing. Maybe they are both enabling each other's recovery, so it may not be horribly bad, but the therapist needs to be aware of who she is seeing, because if things go south, it can lead to catastrophic circumstances for both.
In the meantime, this could be a phase. She might love this friend because she feels she identifies with her, listens to her, and understands her, and so she is unsure of how to express her love or is infatuated and confused about it all. Maybe the friend is influencing her. She may be experimenting, or she may truly be bisexual. Listen to her, be open and accepting to her, and don't judge her, whatever the case may be. That might make her become more depressed and isolated, which then makes her become closer to the other girl and drives her further away from you and her family.
I'd be more worried about her state of mind than who she chooses to make out with. Talking about protection with partners (regardless of their gender) and the dangers of promiscuity is a good idea. She also needs to be involved in activities so she can make friends and get out of that feeling of isolation and loneliness. It might also help create distance with the friend, because if they become very involved with each other and view each other the sole reason for existing, assuming the relationship ends, it could leave them with a lot of anger, emptiness, and suicidal ideation because they have lost their identity and everything around the world revolved on the other person. Obsession and infatuation are very strong at this age.