Hello. If it prompts that error message it means it’s a local user.
Hold down Shift key while you choose Windows restart, then go to Troubleshoot > Advanced Options > Startup Settings, click on restart, then choose option 4.
When prompted for password, enter the admin credential you said you know
When you are in safe mode, hold down windows + r keys, type in netplwiz and press enter.
Then double click on her username, go to Group Membership tab, make it admin. You can also reset her password there
Restart, log back into her user. she should now have admin access.