@OP: I think you meant "respectfully", but isn't there already an afk indicator (the green dot changing to yellow)? Alternatively, you could just leave a status update that says something like "looking for ranked players" or "ranked games only".
Honestly though, if you truly consider them your friend, then that friendship needs to take priority over any game - on both ends. That is, they shouldn't be getting upset about you prefering players of your own rank sometimes, as long as you don't sound like the rager that just shredded them in their last game "yo suck nub" - or anything like that.