If you want to play League of Legends alone, without someone on your contact list knowing, you can do so using LoL Absent!
You can also use LoL Absent with a friend to play duo without anyone knowing. Enter the same game room before and block the contact list and no one will know that you are together.
The application must remain open so that the contact list remains blocked, since closing the chat is unlocked so that the configuration does not persist and thus avoid errors.
The application is very simple, just select the server you are playing on from the list and click the block button or the unblock button according to your interests!