@sandro The first time your friend runs QuickSupport on his device, tell him to keep a careful eye out for a message regarding an Add-on being available from TeamViewer. If it is offered, tell them to follow the link and install the add-on. Once installed, you'll be able to remotely control his device. As 8bitdev mentioned above, not all devices have an add-on.
If they think there
might have been a message, clear QuickSupport's data (Settings>Apps>QuickSupport>Clear Data>Confirm , though Settings>Apps might be different depending on the launcher and Android version). Run the app again now and it'll think it hasn't been run before, if an add-on is available, it will offer again.
If that doesn't help, time to try a different system,
https://alternativeto.net/software/teamviewer-quicksupport/ has some, but I've NOT tested any/scanned for malware etc