IMO if you allow her your friendship she's getting the best of both worlds. My advice would be to tell her something to the effect that your integrity prevents a friendship based on how you've been treated. Tell her you're interested in a productive coparenting arrangement for the sake of your son but that would be the extent of the contact you want to have with her right now. Unfortunately it needs to get worse for her before it will get better.

Giving her what she wants (your friendship) is a prescription for sustained limbo.

Acc

Married 18, Together 20, Now Divorced
M: 48, W: 50, D: 18, S: 16, D: 12
Bomb Dropped (EA, D): 7/13/11
Start Reconcile: 8/15/11
Bomb Dropped (EA, D): 5/1/2014 (Divorced)
In a New Relationship: 3/2015