I always like to preface a tough conversation with "you know I love you, right?" That kind of softens anything you have to say. But you need to make her understand that even though you are sympathetic to her feelings, the situation she is creating for you is unfair and insensitive to you. Let her know that when you invite her ex, it is not a conscious effort to hurt her, but she needs to realize they are seperate relationships and she cannot expect you to drop a friendship for someone she harbors bad feelings for. Maybe she feels that this guy what this guy did is so bad or shows such a bad character that it makes her uncomfortable or angry for him to be friends with you. I know I felt that way for a long time. I understand your cousin's feelings b/c I had them too, but I just had to find my own way through and could not expect mutual friends to choose sides. Let her know you will continue to invite them both to places, and it is HER choice to go or not. Afterall, no one said she had to talk to him.