Why will Outlook not update exchange

I have a customer that uses our hosted solution Server 2008 Exchange 2010 for email access and web dav file share. They have 8 Macbooks connected to the system and they all use Outlook 2011. One user has an issue where he's reading and deleting emails within Outlook on his mac but it isnt updating Exchange which in turn means his iphone isnt clearing down the read and deleted emails. If i log on to the owa mailbox i can see that the emails havent been read or deleted. We have this issue for one user with the newest MacBook. It first occurred 4 months ago and i ended up removing the account and resyncing the whole lot which took a few hours due to the size of the mailbox. I've right clicked the folders and told it to resync but it remains the same. Does anyone have any help they can provide please? thanks

The user doesnt want to rebuild the outlook account. Am i missing somthing obvious

MacBook Pro, Mac OS X (10.7.3), Outlook 2011