I was just trying to verify that when you say "logged in", that you were referring to users who have a remote desktop session on your server as opposed to mapping a network drive to the server.
I did not see anything in the group policy documentation that looked like it might allow the access that you desire.
https://www.microsoft.com/en-us/download/details.aspx?id=25250
I have heard of this 3rd party tool that allows a user to run a program as an administrator account. I don't know if that would work for task manager or not.
https://robotronic.de/runasroben.html
Have you tried having your manager access the Terminal Services mmc snapin's?
http://woshub.com/how-to-run-tsadmin-msc-and-tsconfig-msc-snap-ins-on-windows-server-2012-rds/
In addition, there is no Security tab in the tsconfig.msc console in the RDP-Tcp connection properties. This tab was handy for granting permissions to the tech support team (non-admin group) to connect and manage user sessions on RDS host. I use the following command to give a specific group full control over the RDP sessions on the server:
wmic /namespace:\root\CIMV2\TerminalServices PATH Win32_TSPermissionsSetting WHERE (TerminalName ="RDP-Tcp") CALL AddAccount "woshub\helpdeskteam",2