Change computer Apple TV uses to connect to iTunes
On the Apple TV:


Go to Settings > Computers
Turn off Home Sharing
Turn on Home Sharing (it will ask if you want to use your Apple ID or connect using a different one)
Select the option to connect using another account
Have her enter her Apple ID and password in


Of course for this to work, she will have to enable Home Sharing on iTunes on her Mac. For directions, see Apple's guide on setting up Home Sharing.

As for Netflix, you should just be able to log out and log in with a different account.