She has absolutely no interest in him anymore, and is over the relationship. She was the one who broke up with him, but he wanted to remain friends. Might be important to note that she's not crazy about staying friends with him, but when he insisted she didn't want to be rude and a "bitch" for saying no to friendship.
I personally would think you'd be ride or a bitch for denying him wanting to remain friends with him! I personally don't keep exes around AT ALL not even as friends. For me it may be a bit awkward that you chose to remain friends even though you didn't want to because to me it's a small conniving way of him having access to you albeit in a very minute way. More importantly, it would be how I see the interactions between you two before I would do or say anything more since a relationship needs to start off with trust 'n' communication.
I will still date with her, but will remain it casually. When she's comfortable with me, she will bring it up being exclusive. Her boyfriend is not my concern anyway since they have been already break up. If it was meant to be going back to his side, well, i will let her go. Still have plenty of options out there.
I obviously wouldn't like it, but if she only saw/met him occasionally or as a part of a group of friends, it wouldn't be a problem. But if they were meeting up, spending a lot of time together then that would definitely be a problem.