Stupid forum timeouts eating my detailed reply. So now you get the short version.
And, if it did cancel the meeting for everyone when one person declined, that sounds like an Exchange bug.
In fact, I can confirm that this exact same behavior can be reproduced in an all-MS IT infrastructure. We've started seeing this problem the last couple weeks here, and it seems to stem from interactions between Exchange Server 2003 and Exchange Server 2010 [everyone is on Outlook 2010, but we're in the middle of a server migration, so some people are still on the older servers].
I personally confirmed that the person who declined did so on her computer (not on a smartphone), and according to the IT dept, her account is on Exchange 2003, while the other affected parties that I checked on are on Exchange 2010. (I didn't check every single one of over a 100 included people.)
Outlook/iOS/Exchange Server problems also aren't new with iOS 6. We've had a standing corporate policy not to interact with meetings via iOS for as long as it has existed--it just used to only screw up the calendar of the person who did it, not others.
If we can't be rid of Outlook, I wish MS and/or Apple could sort out these problems. Given that, according to MS, Exchange Server can't reliably manage recurring meetings with more than about 13 recurrences, I'm inclined to suspect that it's MS at fault here, but I won't be at all surprised to find out it's Apple or both of them.
(In researching the pre-iOS6 problems, I found more than one person who claimed to have traced the problems to a bug in how Exchange Server 2003 implemented the ActiveSync/Exchange protocol. They said that the bug was still there in 2007, but fixed with Exchange Server 2010.)