We all had this situation where our opponent have this fide master friend and just happen to stand next to him, whisper him things while pointing your chessboard.
I've never had the guts to call it out (that would make a mess out of no proof accusations) but that usually mess up my game as my anger take over my concentration.
I'd like to know if you've met this situation and how you dealt with it, or what would be the official way to deal with it.
Please consider that calling out referee might not be the best solution : as I said, there's no proof, and it's easier said than done to call a cheat when there's no evidences.