How would you deal?

My ex and I split up in 2012. We have a 5 year old son together. After we split up he engaged in a relationship with my best friend of 6+ years. I instantly stopped talking to her after that. Then I tried to get over it, because he is an ex. But then it was just extremely awkward. Everything she loved about him, she once told me to leave him for. Well, they recently got married and now she handles anything and everything that has to do with our son. For instance, my son came home from their house (he's allowed 2 hour visits once a week, court order) and my son an 3 bruises on his arm. When I called to ask him what happened, she kept replying for him in the background. Well come to find out, her son was throwing toys and happened to hit my son in the head and yelled at him. When my son said her son grabbed him, she flat out called my son a lier, in which unfortunately my ex also said to. Later, about 2 weeks after I brought it up and he admitted to her son grabbing his arms. He admitted this when she wasn't around. When I want to get to the bottom of things I have to talk to her, and my son is lying, or I'm not listening to her. So, how would you ladies deal with this?