Ok, let me try and understand..
Your wife used to have a separate iMac of her own, but now she has a user account on your joint iMac? If so, there are several ways of doing this:
First, make sure she has authorised iTunes on the new Mac with her Apple ID then,
- Simply copy her entire iTunes folder (from the Music folder in her old iMac) onto a memory stick or other external drive, then assuming her new iTunes doesn't contain anything, simply copy the iTunes folder from the drive into the Music folder in her new account.
or..
- Sync her old iTunes onto an iPhone, then sync the phone in iTunes on the new Mac - choose the sync setting that adds new tracks without deleting existing ones.
After doing either and making sure the new iTunes has the music..
- if she is no longer going to use the 2011, deauthorise iTunes on that computer
- otherwise leave the 2011 as is, then there will be two machines authorised for her iTunes