How would you feel?

My ex’s girlfriend started blowing up my phone, ranting about my 6 year old bullying her kid and that he’s an asshole because he doesn’t listen to her and is not welcome in their home anymore..and my sons father, is taking her side. if you people knew my son you would be dumbfounded because he really is the sweetest kid. Not just because he is my son but his teacher sends me letters saying how proud she is of my son cause he helps the autistic kids in his class learn how to play certain games and plays with them if they’re by themselves at recess. and that he’s so kind in general. Also the summer program and my friends have babysat and thought he was so well mannered and well behaved taking turns with others and all. Like I said he is 6 so of course he does try to get away with leaving his room a mess or whatever else 6 year olds do to misbehave. Welll this chick has had it in for my son since day 1. How would you handle this?