Hi GottaMoveOn! Friends... with an ex... I'm not saying it is never possible, but. When it is possible it's because a lot if time without contact has passed and all feelings other than friendship (whether positive as in love/wanting them back or negative as in angry) are gone. Are you in that place yet? Do you really want to be friends to be friends or because you're not ready to let go? This is what I would ask if your ex didn't have BPD.
Seeing he does have BPD, IMO it isn't possible. BPDs have strong holds on NONs so chances are you want him back now or will in future as more than a friend. Or he will want you back because he needs supply. You'll end up recycling and losing more of yourself each time. Be careful not to get sucked back in. Go NC. Stay strong Come here often, it helps, a lot