iPod touch resetting
Easy enough...

go into Settings -> iCloud and sign out of his account there. You may also have to do the same in the Message -> iMessage settings.

Once that is done you can use her email for the icloud and iMessage account.

If you do that and are still getting his messages use the Apple Support page for de-registering your account on that phone:

https://selfsolve.apple.com/deregister-imessage