You can be friends with an ex but only if you both know their is no going back to a relationship status.Im very good friends with an ex and his current GF,She sounds like someone who wants to keep all her doors open and this is never good in a relationship with another person.If it doesnt work in a relationship basis it should not be reopened and closed at the others whim.Sounds like a physical/emotion tie again until she wants something she thinks is better sorry .
A mistake is always forgivable, rarely excusable and always unacceptable.
Robert Fripp