Share Itunes Library with all users on same machine
The trick to doing this is to have your iTunes library stored in a location that is accessible to both accounts (i.e. both users have read/write access), and to then configure iTunes in both accounts to use the same library.

My recommended approach for doing this is to use an external drive connected to your Mac and then configure iTunes within each of your Mac accounts to use that library.

Follow these steps:


Ensure iTunes is not running
Within Finder, move your iTunes library folder (by default this folder is located within ~/Music/iTunes/) to a location accessible by all user accounts. As mentioned above, my personal preference is to store it on an external drive connected to your Mac.
Hold down the option key and launch iTunes
You should now see the Choose iTunes Library prompt (see below)





Click on the Choose Library... button
Now navigate to the location you moved the library to at Step 2
iTunes should now open up and work as it's always done previously
Quit iTunes
Now log into your wife's account
Hold down the option key and launch iTunes
Repeat Steps 3, 4, 5 and 6


After doing the above you'll be able to access the same iTunes library from either user account.

IMPORTANT NOTES


If you don't already, ensure you have a backup regime in place (e.g. using Time Machine)
If using an external drive (as per my recommended approach) ensure you include this new location with whatever backup software you use (so that it's getting backed up)
There are a number of reasons why I prefer using an external drive for my iTunes library. Some include:


It frees up space on your main boot volume
It results in faster transfer rates (in the event your internal drive is not an SSD)
I like to keep all my digital media on the same drive (i.e. I also use the same drive for my Photos library)
It makes your iTunes library (and others if they're on the same drive) easy to physically transfer (in the event you need to)