Depends on how your ex feels about you, if she has strong feelings about you her friend would be playing with fire if she went out with you. Also how long as it been?
I mean if it was a week or something i thnk thats a little jacked to go after her friend right after you break up with her, but hey thats me. You should find out where you two stand. If she no longer cares, go for it but explain the situation and if you wanna be friends with her, well dont count on it if you date her friend. Now if she does care about you then i dont think its a good idea, not only would she be upset with you but you could mess up a friendship between the two of them.
Remember it was a group of professionals that built the Titanic but an amateur who built the Ark
~~~~~~~~~~~
Love isn't finding the perfect person, its seeing an
imperfect person perfectly