What About Your Ex-Girlfriend?
That relationship is over. Why would you give her veto power over your future?
Don’t ask permission. Sure, you or her friend should be the first to tell her, but only after things have developed. If you ask her permission and she says no, you’ll create an even bigger problem for yourself if you go for it. Plus, once you tell her you’re interested in her friend, she’ll know who to go to to sabotage your chances.
Meanwhile, your ex’s friend will likely want to ask for permission, and just as there are situations in which you wouldn’t want one of your friends to date an ex of yours, your ex’s pal might not want to date you if it risks that friendship. But that has nothing to do with you.
The bottom line is, if you’re interested, go for it. It might not happen, but there’s nothing wrong with trying—and, ultimately, any issues between your ex-girlfriend and her friend are theirs to work out, not yours.