C., I hear you, I really do. I was a junior high teacher for over 10 years and this is more common than you might hope. All I can do is tell you what I know:
I know that forbidding her to see him is counter productive. Think of it this way, if I said to you "don't think of a pink elephant" what do you do? You think of a pink elephant right? Tell her what you want, not what you don't want. Redirect her actions, her activities, her thoughts. Start reading a book together, join a class together, take up a new hobby. Ask for her help around the house FUN stuff only, don't let her think it's any sort of punishment. Maybe start learning some new recipes TOGETHER. Redirect her time.
I know that she loves him, she does or she wouldn't say so. And whether or not you believe her is not the bottom line, it is the fact that SHE believes it. Let her love him. This is a great opportunity for you to teach her what real love is all about. Tell her wonderful stories of you and your love/s or ones in your family. Give her examples of how it "feels so right" and "makes you so happy". Guide her to have the relationships you know you want for her and not to settle for "what might be".
I know that you know exactly what she is going through, but that was 20 years or so ago, of course you don't know what she is feeling... "so she says". And she is right. You can look back on your experiences with the wisdom of your current age, she doesn't have that yet. You will never convince her that you do know or understand just by using words, you will have to share with her your experiences. And not in the "ewww gross Mom" way, just the casual "oh I remember when" way.
I know that when you let her grow and be her own woman, she will make the right decisions, but you have to be willing to let her make mistakes. Imagine if she did not have this experience now at age 14 and she had it when she was 25 and ready to marry. Then what? What could you do then? Let her live.
C., help her grow through this. Please stand beside her and reach out a hand to help her along. I know for a fact that it is easier to reach out your hand and guide someone than to push them from behind or worse yet, pull them along from in front.
B.
Family Success Coach