Hi Heathal10,
While it's possible to create an account for a child for the purposes of Family Share (Create an Apple ID for your child) there is no function to force an existing Apple ID to accept an invite to a group. Control of the device and/or the Apple ID would be necessary to complete the setup for Family Share or to enable Screen Time restrictions in the situation you describe.
From the article Leave Family Sharing:
"Remove yourself from a family group
Any family member age 13 or older* can remove themselves from a family group. If you have Screen Time turned on for your account, the family organizer must remove you. "
This would indicate that restrictions were not in place prior to the individual removing themselves from the group, and that the removal was permissible with Family Sharing rules or features.
Regards.