Your friend's ex's friend is the obvious choice to date. She is not coming out of a relationship that just ended. She gave you a lot of positive signals even though you'd only just met. BUT, she is best friends with the other girl, and that is going to cause at least a little friction.
But from your account, your friend's ex doesn't seem to have any particular interest in you, she's just in the post-breakup blues and looking for attention. Who knows, she might find some dude to hook up with this very night.