I agree with your efforts so far.
It is so tough, as a parent, to see you child in a relationship with someone you don't think is right for them, but it's too easy to make things worse by trying to intervene. Fortunately, most young romances last long. Circumstances most often pull the couple apart. There is a lot of tongue bitting you need to do, to keep from fighting with her about him. Equally important is how you react when they do break up. You may be very tempted to tell her how glad you are that they aren't together anymore and he was never good enough for her. This can cause as much hard feelings as comments made while they are still together. It's far better to treat her as though she just broke up with a great guy. Just listen to her and tell her you're sorry things turned out badly for her.
I hope this is helpful.