Separation work

So long story short, my kid’s dad has recently got divorced from his wife. He has a new fiancé, which only 1 of his 4 children like. My oldest literally cringes when he has to go over there and his younger 2 kids throw a fit to their mom(the ex wife) because they hate being there with the fiancé. He works weird hrs and sometimes he is gone for 24hrs straight. Well he has always been an absent father until this girl came along and now he Wants to dad if the yr and wants his rights. He took me back and got his rights established. Which is fine but this woman has a weird relationship with my 14yr old son. She only does for him, like takes him lunch to the school. He has started acting out and treating me like im nothing. He now wants to live with his dad when he never wanted anything to do with him before.. idk if she has groomed him or what.. everyone has said its strange.. what can i do to break whatever they have going on with her.. but not break it from his dad? I want him to have that relationship with his father .. but only with her