Sounds like you have her thinking about it. Bravo! This is the best way to start.
She will be, after all, the one to end her involvement with her ex-boyfriend.
Not you. This is something you can't make her do, she will see, and act, as she learns and grows.
Ask her what the prize is, she's trying to get. If she says "I don't know"
Ask her to look deep into herself and ask herself why is he so important to her.
If she replies "He has no friends"
This is not an ansewer, It also has nothing to do with her. His ability to want or make friends is not her responsibility. So why does she think "this" is?
This is the best place to start.
It is more important to learn why we do something, that to just do it.
Also tell her that she does know why, deep down inside herself and needs to explore her own "Whys"
She can never help anyone untill she learns to help herself.
The same is true with her ex-boyfriend.
Getting her to explore herself with you can draw you closer together.So remember that if you get mad or yell, you have gone to far,and can do more harm than good. Take you time, hold her hand, look into her eyes and speak from the heart.