boyfriends ex accused him of rape

hey my boyfriends ex uses their child as a weapon when they argue he cant see his son and when he does see him its when she says so she's not even letting him see his son on christmas. the other day he called at her door but no one answered so he looked through the window and there she was on her knees with her new boyfriend having sex while the little boy was sleeping on the sofa beside them. my boyfriend went nuts obviously and he admitted that he did kick her but now she's claiming he raped her i want to know what evidence would she need to prove it in court and if he did do that then would the new boyfriend who i assume was there if they were in the front room going at it then would he not step in and help her....... i need help and advice