You can utilize the in game report features, on the player menu. Just click on a player's name in the Roster screen to bring up this menu.
That is the best way to notify Bungie if you believe a player is cheating in game.
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I did this of course, but I was concerned that this does very little and Bungie might never be made aware of it properly, so I'm just being extra safe by posting here also to see if there's any other way of shedding light on it. :)