The app/add-on could be great for keeping other ppl away from some websites and content, but it too easy for your friend to pause the block when he really want to see those sites again (late on sleepless night). I think he can just put his password in and see any website he want.
Maybe a good method is to set up the app/add-on, create desired filter, then lose the password. Maybe even better is for his friend - you - to decide password so he can't see those site unless you give him password. It often take a good friend to overcome temptation.