I think that Ben said it as another way to get under Jacks skin... He somehow knows how to push peoples buttons.... Sarah is a sore and painfull subject for Jack... He was going to use Juliet to get close to Jack so that he would trust her... Ben said it when Jack was/is starting to trust Juliet.. I think he knows she is a traitor and Ben wants Jack to not fully trust her.. Make him realize she was just a tool to get close to him