Does anyone else's ex weaponize the kids' emotions?

Every time I have to enforce a boundary (like, my rules at my house) my 8yo comes back and says 'mom says you're being mean.' Or 'mom said you don't actually want us here that much.' Like I get that custody is hard but filling his head with stuff like that feels... not okay? How do I handle this without badmouthing her back? Feel like I'm always the one taking the high road.