I need to let a second user configure a part of my server and I wish to both log what the user is doing with terminal and view it in realtime so that I can react if something goes wrong.

Know of a good and easy way to do this?

use the "screen" command to create a screen server on the client machine (server) and connect to this screen session from your computer using ssh (ssh to the server) then you can try to use "tee" or I don't know what to log everything, but it will mostly log control sequences, which are not really useful to you.

An other solution would be to use git to store various revision of the file and hook inotify to commit every time the file is closed. This should output you a nice diff all modifications. This plus the real time screen sessions and you should be in business. This will not log the commands executed, but there is .bash_history for that (but it can be bypassed or edited afterward, ot even never loged in the first place if bash is killed).