How can I remove/transfer a child on my family sharing account so that he can be added to his mother's family sharing account?

I added my 11 year old nephew to my family sharing account. His mother has now created an account and would like to have him removed from my family sharing account and added to hers. When she tries to send an invite to him, she gets a message stating he is already part of a family group and he must be removed from it before she can add him. When I try to remove him, I get the message that he must be added to another family sharing account before I can remove him from mine. Any advice or tips on how to have this process done?