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As long as you are broken up you don't have to tell her anything because you aren't a couple but if you are considering a possible reconciliation then it's important for you not to go on this date if you truly want your ex back but if you aren't sure then why live with regrets go out with the other woman and see if the two of you have anything in common. When you are trying to reastablish a broken relationship dating someone else isn't always the best idea, it will only make things more complicated than they need to be.
If your ex already has trust issues it will only make it worse instead of better, you never battle trust issues with more trust issues. I would suggest just concentrating on getting your ex back and not dating this other woman.