Depends. Sometimes exes get jealous, even if they say they have no problem with it. Just depends on how crazy she was about you and how bad the break-up was, and how close she is to her friend, and if she's still friends with you after breaking up. Best thing to do would be for all three of you to sit down and discuss each of your feelings on the matter. If there is any chance it would cause a problem with the ex, you're probably better off steering clear.