My wife can stream the music on her iPad2 to our home theatre via Apple TV. We have one iTunes acct for our purchases, etc..When she goes to visit family in Ga. Is there a way she can stream the same music via her dad's home theatre/Apple TV set up. I'm assuming there's an issue related to our iTunes acct vs her dad's iTunes acct.
From an iPhone/iPod/iPad you can stream music to any Apple TV without needing to log in in any way. So as long as your wife doesn't want to copy her music onto her Dad's system she should be fine.
One Computer must activated for a specific iTunes Account. Once the Mac/iPad/iPhone/AppleTV is activated, you have to wait 90 days to switch the account.
For example: If your Dad has activated iTunes Match, he has to wait 90 days till your wife can activate iTunes Match on the same device with her account.
The solution would be to carry around your iPad and stream it from there.
There is no sharing between iTunes Accounts. So currently you have no chance to transfer all music from one account to another.