I have been the prop of a woman with a boyfriend.
She would use me to humiliate him, either indirectly with words by things like calling him the "third wheel" through text to me, getting a response, and then showing him the conversation, or directly by being physical with me in front of him. Like when I reclined in a chair at their place with her, him and another girl, the girls both came to rest on me - and she just laid herself right on me, covering me with her body just like the bed cover she also brought over after a while. He asked her to come where he was sitting instead but she wouldn't. Or when she went around flashing her t1ts to a few guys including me at a party, with him in tow and because he was in tow, which was years before the chair thing and I think was during an "argument". She would talk him down when with me and he wasn't there, or even jab him when he was IIRC.
You would have thought she would cheat on him with me, or at least break up with him, but she never did - and now they have two kids. On facebook picture captions she says how much she loves him.
She's a year older than me and her boyfriend is two years younger than her. I met her in school at 16, they'd known each other for a while before that, they then got into a relationship after a while and this stuff started happening. I went NC on her a good while ago because while I always thought she was fun to be with (probably cause she was one of the first girls I really "got to know" in a new phase of life), she was just too manipulative and her being in a relationship gradually deadened the fun. One reason she brought up for her behavior, directly or indirectly (don't remember) was his relation to an ex - but she continued, and indeed did the worst of her behavior toward him with me, long after he had ceased contact with her.
She was also the only child of a mother who raised her, and a father who never wanted to have her and abandoned her before she was born I think it was.