So this is clearly a problem. Let's fix it for Blizzard: how can we make sure everyone has five chances a day at summoning the Headless Horseman (one per day per group member) without ripping off people for helping their guildies or friends? Basically, we need a clear way of seeing whether someone has a summon or not -- maybe a debuff that resets at midnight every night, or a changing number of candles on the pedestal based on how many people in the instance can still do the daily quest? Right now, there's no accountability at all, and with droprates the way they are, getting screwed out of another chance at the HH (or even four chances, in the worst cases) can make the difference between having the achievement and not.
Any other good ideas on how to solve this problem and keep players accountable? I'm sure players with less than upright morals have no problem claiming their brother used their summon or lying to steal the summons of others, but it would be a lot more fair for everyone if the possibility for scamming wasn't there at all.