I can't believe I am about to say this but...in a weird, strange way I actually agree with schultzz.
We have all dated those people that we NEVER should have. You have such a bad break up with them that after the relationship has ended you feel you have absolutely no obligation to them anymore. You are not friends so respecting each other is out of the question. I'm guessing that's where your big's ex is at.
A few years ago, my friend, "Jaime" and her boyfriend had a horrible break up. After it had ended, he started hitting on my other friend, "Stacy", and telling her that he never liked Jaime as much as he likes her. It was a big mess. Of course, we thought the guy was a sleazeball but, at the same time, he had no obligation to respect Jaime. But, Stacy did have that obligation. Stacy told the guy to get lost and he hasn't spoken to her since.
I would never hook up with an ex's friend but I wouldn't say that's out of respect for my ex (if the relationship ended on bad terms). I would say that it would be more out of the fact that I would not want to get involved in a love triangle like that!