Try this:
Method A ( Worked for me ):
- Go to LoL's install folder ( should be Riot Games, inside there are bunch of folders like Config, Cookies, DATA etc. if not go to method B.)
- Create 2 new folders. Name one "League of Legends" and the other "Riot Client".
- Now move the whole game to the "League of Legends" folder.
- Open the "League of Legends" folder and launch the game through the "LeagueClient.exe" file.
- Login in and then go to settings ( upper right corner > gear icon ) > General > sroll down and hit "Initiate Full Repair".
- That's it. Your launcher should update and your friends tab should work.
Method B ( worked for some1 else ):
- If u already have the "League of Legends" and "Riot Client" folders inside the Riot Games folder, delete the "Riot Client" folder.
- Open the "League of Legends" folder and launch the game through the "LeagueClient.exe" file. ( Should promp an error but it should be fine ).
- Login in and then go to settings ( upper right corner > gear icon ) > General > sroll down and hit "Initiate Full Repair".
Let me know if it helps :) GL.