What do you do when a parent uses ADHD as an excuse for a child's bad behavior?

My dad and stepmom adopted a boy from Russia 5 years ago. It is my dad's 5th child, and my stepmom's 1st. All of my father's children from his first marriage are over the age 17, and my adopted brother is 6. Over the last several years, my brother has had multiple incidents of violence, like kicking, biting, hitting and hitting with items (like shovels). He's been kicked out of all of his preschools for disturbing the other kids. He would talk incessantly and was openly defiant about everything. Right now he's violent, doesn't listen, and most requests result in full fledged tantrums. I know that he has ADHD, and I certainly respect that it makes him more impulsive and makes it defficult for him to listen, but I also know that his mom is the first person to blame someone else for incident's with him. When I babysat him we were playing catch and he didn't catch the ball so it hit him (soft nerf ball) and he told her I threw it at his face and she actually entertained the idea that a 21 year old would actually throw a ball at a 5 year olds face. So, when he acts up, and when he if freaking out or getting in trouble at school, is it really because his ADHD isn't under control or could i be that she doesn't discipline him? I feel like every time I comment on his out of control behavior, it's because of his "condition" or his medicines. I know other kids with ADHD and their parents don't blame the condition for their every lash out.