Good Afternoon, I was wondering have you tried talking to her. Not a talk where you are judgmental or letting her know how uncomfortable it makes others feel. More of just a women to women talk. Use that time to see what she likes about him, why she is desiring to hang on him, and just overall what she is feeling about the relationship. I am betting she feels insecure, and that leads her to feel like she has to hang on him. Through a talk where you dont judge but just talk you could determine this.
In return if it is a security thing, then you can talk to her about how she doesnt have to be insecure and give her advice. At that age she doesnt want you to tell her what to do but give her advice like a friend and talk to her like one as well. I know that you are the mother and she needs to know this, but it doesnt hurt to be a friend at times as well. This will open her up to coming to you when the problems get bigger.