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As others mentioned, the iLoL client (boompje link) works. It's not a native Mac app, so it can be wonky at times, but it usually works quite swimmingly.
Have your friend bookmark that site and play a quick custom game after each patch to make sure things still work (in case a patch broke something in the iLoL client - if it does, check that site for a workaround).
If you're using skype/vent/etc to talk, make sure your friend fires that up before starting a "real" game - the iLoL client doesn't minimize/hide/etc very easily if playing in fullscreen so it's tough to answer/call once the game's running.
CMD-OPT-ESCAPE to force-quit the thing if the client misbehaves and your friend needs to kill/restart the game.