I think it's up to you on whether or not you want to say anything. On one hand, she DID tell you that she was seeing your old boyfriend... but it doesn't necessarily mean you have to return the favor. After all, you dated him first, and she didn't even date him for that long. Ultimately, if you don't want this to be any of her business... it doesn't have to be. If you feel like she should know, however, then I don't see how mentioning it would be a problem.

Why would you stop seeing this guy? Has he done anything that warrants breaking up with him? Some people are going to say "ex-boyfriends are always ex-boyfriends for a reason"... so if you feel like there's a reason why the two of you shouldn't be together anymore, go ahead and break up with him. If he's changed, however, and the only reason why you're thinking about breaking up is because you're concerned about this girl's feelings... don't. Oftentimes, there will be ex-girlfriends who still aren't over your current boyfriend. You can't let them dictate whether or not you date someone.