As in, i'm on voice wtih my friend and can't see him/vice versa
Friends list does bug out occasionally. Just give it a few minutes and it should fix itself. Otherwise, log out and back in and it will be up and running in a matter of minutes.
I had to delete and re-add everyone on my friends list, because it was completely broken. No clue if you have to do that.
© 2014 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.