I need advice

OK. My brother and his wife split. She was mean to his kids and degrading to him. Stuff like she would try to get him to hit her by saying "are you mad enough little boy" yada yada. And to his kids she would say they were ugly. Bitches. They are 5 and 4. So much more to it though that she did. Well. Now they have split she has plastered on fb how he is a cheater, woman beater, and now she is saying (anytime she gets the opportunity) that he just abandoned the daughter they have together and is screwing everything he can find. None of this is true. He doesn't even have a car to run around with other women because he left with nothing. Not even his car. His clothes.. nothing. He bought the baby stuff for her birthday and diapers and she literally threw it back at him. She is running his name through the dirt and I'm ready to put her in her place, but should I? Or should I just stay out of it? His new woman is driving him back and forth to our house since he works with my husband and they ride together. Just in case y'all are wondering about that