First, launchers are apps like all other apps you can install. Which means: per se, there should be no problems -- but they of course might have bugs and, in rare cases, side effects. Again, this is not limited to launchers. So far for the "generic info".
Now specifically for the launcher: After having installed a launcher (Nova should be fine, Apex might be another choice close to Nova), when he presses the Home key to switch to the home screen, he will be asked which launcher to chose. So still he's got the choice to use the MIUI launcher or his "custom one". On every press of the Home button. Until he decides to always use one of the two, and marks the check button to remember his choice ("always use..."). Now on Home key press, it's always that launcher being started.
If at a later time he wants to switch back: still no problem. The "decision popup" will come again if one of the following conditions are met:
- he installed another launcher
- one of the installed launchers receives an update
- more than 2 launchers were installed, and the current default launcher is uninstalled
- in Settings→Applications, the settings of the current default launcher are opened, and the "clear defaults" button is pushed
- some "default apps" manager was used to trigger it
So in short: I'd expect no trouble. Some special features of the MIUI launcher might not be available in other launchers, some specific MIUI widgets (specifically built for the MIUI launcher) might not work with the other launcher -- but if he wants, he can always return/swtich or even revert his changes.