In my most recent session a player in my group who is a Synthesist Summoner attacked a burning skeleton. I ruled that the Fiery Aura activated with each natural attack, and so with his full attack action each natural attack resulted in 1d6 points of fire damage. They became irate and demanded that is not how auras work, and that it should only activate once with the first attack. While I think he's wrong on this, I haven't found a definitive answer online. Could you please tell me which of us is right?
The description is simple -- anyone who attacks it with a natural attack takes 1d6 damage. You attack it, you take 1d6 damage. You attack it again, you take damage again. There is nothing to support their hypothesis about 1/round limitation. I suppose they misinterpreted the statement in the previous sentence.
Now what I would truly like to point out is that you have no duty towards your players to play the monsters exactly as in the rules, even if they were right, which they aren't (assuming that isn't what you agreed on beforehand). You may add a paralysis breath attack to the skeleton, and they cannot complain, because they have no control over the world or the monsters. Even if the skeleton was dealing cold damage, it is ultimately your decision what his capabilities are, as long as you are internally consistent.