F1ach: Your relationship isn't honest. You are "fooling around" behind the scenes.
If she cared enough for you she would dump his sorry ass. Because of this be careful how you handle the situation, she could just be using you as a crutch and until she makes that break, you will never know for sure.
I knew a beautiful girl in a similar situation and the amount of times she went back the scumbag was unbelievable, it was like she couldn't live without the violence or the adrenaline or something, it wasn't the tenderness and compassion for sure.
Sorry if that sounds uncomfortable, but its an objective view.
Good luck however it turns out because apparently you are very fond of her.
^^ is what I have learned from similar situations. It is easy to pass judgement on everyone involved in this and I bolded the part that has been most relevant to my experiences. Starting a relationship out of dishonesty seems to
have that relationship end in dishonesty.