How do I be supportive of my awesome longtime girlfriend and her insolent 15YO son?

You sound like a really good and caring man. You are unusual in that you know the rule that you are not supposed to discipline him. You are funny for saying that you are the one who needs the therapist. Aside from what you have already done and that is to suggest that she and her son see a therapist there's nothing else you can do. But she is so very wrong in saying "he won't go" I would want to ask who is the boss here the boy or his mother. It seems as if he is the ruler of the roost. He has given up on him and that is a shame. It is her job to get him into therapy whether he likes it or not until he is the age of 18. By then obviously you know that will be two late for him. She has a responsibility and an obligation to see that her son is raised with rules and as you say he has none. I am so sorry for the situation that you are in and it is certainly kind of you to like him despite his lack of discipline. Maybe you could ask her who is the boss her or her son and let her think about that. He is being raised in such a way that he will be abusive to the women in his life and that's the least of it. It seems to me that you can only continue doing what you are doing and that is being kind to him despite all and showing him respect that he refuses to show. He is going continue to be trouble for her as he becomes an adult. Just look forward to the day he goes to college or gets a job and will be out of your way. You should be proud of yourself.