Well if you give her a relationship you will only be the rebound guy that she will leave you for later once her emotions settle or the ex takes her back whichever comes first.
So you should get the idea of having a genuine relationship with this one out of your mind.
However if you are up for a fun fling go for that and just tell this girl what she wants to hear.
Put your own personal time limit on the interaction though and be sure to end it when your time limit you have given runs out for the relationship.
With any luck she will transfer the longing for her ex boyfriend over to you and you can put all this misplaced emotion of her's to good use by periodically using her for sexual favors.
