I too thought first of restraining order to make sure he doesn't re-enter your life.
What would you get out of this "friendship?" What are your motivations?
For me afriendship means I am on equal footing with the other person, that we can contribute equally to the relationship, that we both get something out of the relationship that is good and healthy and fun. I don't see getting any of that out of a friendship with my xAH. I don't see where you would be getting any of that out of friendship with your xABF.