It's ok to cut ties with an ex.
Mark just broke up with his ex for cheating on him and knows that she has few to no friends.
Mark wants to move onto someone else.
Mark cuts off all contact with his ex and focuses solely on finding someone else.
Mark feels bad for his ex, so he stays in touch with her strictly as friends and pursues other romantic options.
Mark's ex has to further reap what she sowed when she betrayed him.
Mark's ex does not understand how serious her actions were.