I don't think this is even the discussion that needs to be had. The real problem is that if you tell someone they are your soulmate, your reason for living, your world, to then say you don't even want to be friends with them because they have slept with someone else, when you had split up, makes no sense. Does the act of sex negate all that went before? Does sex reach into the past and remove the closeness you previously had? If someone is your soulmate are they not still your soulmate when you are not together? Can you not still help each other?
Maybe it is just hurt getting in the way and when the hurt goes through it's stages the friendship will be able to be be recovered?