Can you use the time at the park to show the mom that the child is being impacted negatively such as having the group do something and then when she protests saying something like sorry XXXX but mommy and I need to have an adult conversation while you play with your friends in the sand and then turn to mom and talk about anything. Say let's go sit over here on the bench where we can see the children play. Then sort of steer her away from the child - gently of course. Every time the child comes running be the one to speak to the child saying you need to go play with your friends. Mommy can't always be your playmate things like that. In time the parent may see that she is having a negative influence on the child.