How do I handle this?

How do I deal with his baby mama?
My husband and his baby mama have a 4 year old son which I absolutely adore and we get along very well. Well recently his mother has been really stepping outta line. She has sent his messages asking some really nasty questions then will tell him he’s not doing his part with taking care of their son. But yet the days she has him he’s with her mother and hardly spends time with him. He’s started saying that she doesn’t like him. He’s said something to her about the inappropriate messages but never when she doesn’t see her son or belittled him when we pay for everything for him make sure he goes to school has everything he needs. I’m not mad about having to take care of him without her help at all I’ll support that little boy forever. But how can I remedy this without feeling like I’m over stepping my role?