your grand daughter will be alright. While it may confuse her right now because she is moving away from the daddy she claims she will get use to it. If the "daddy" is going to stay in her life and mommys than there is really nothing to worry about, they will still have their time together and she will still beable to see him. Maybe you can suggest that the boyfriend help her more with her schooling before she moves out, that may help alot, or maybe you can help her study more.
It was hard for my kids. I was a single mom for 12 years. Last year I moved out of my ex boyfriends house into my own and my son was having troubles with it (he never called him dad or anything but was still having trouble), about 2 months ago I got married to a wonderful man after only knowing him for 7months. My son started calling him daddy by himself before we even thought about getting married. I guess kids have a sence of things that are good for them. Maybe your daughter needs to listen to the little voice coming from her daughter. She maybe having trouble in school because it may not be for her any more.
Theres two ways this could go. Try talking to them and find out what all three of them want. They just maybe able to work things out that will be good for all involved.
I wish ya'll the best of luck.