Be upfront about it, the worst thing you can do is be shady about this topic.
It does depend on a couple of factors though:
How long were they together for?
How good of friends are you and Joe?
And Possibly, why did they break up? (Cheating? Personality?)
You wouldn't want to decide whether or not you would rather risk the friendship over her or not.
Be honest though, tell him you only found out recently that she liked you, and that if he would be alright if you two tried dating. If he's mature about it, and accepts that you were a good friend to tell him, then he should allow it. If he blows up and gets angry, it is better to not jeopardize what you have. At that point let him cool of for a little bit and see what happens, perhaps he'll come to his senses.
I am sure that she'd understand if you both couldn't date right that moment. Don't expect her to wait for you until he gives the go ahead though, as if she's looking for someone else this fast it's likely that she will not wait.
Ultimately it is up to you. There are many women out there my friend, but close friends come rarely.
-Ki-