Change of user for the Systems Photo Library

Photos for Mac has been designed as a single user program.

Each user should maintain an own Photos Library and you occasionally share special photos that might be interesting for others.

Whenever a user opened a Photos library, even if you only want to browse it, the library will be modified and you need the read/write access to the internal files. The file ownership of the internal databases will change. That is why the library needs to be on an external volume or in a separate partition, where the "Ignore ownership on this volume" flag can be set.

Tony already pointed out, that only one of you should be logged in, if you are sharing a library. The reason for this is, that the System Photos Library is special and always in use by the Photos background processes, while you are logged into your account, even if you are not working with Photos.

if another user wants to open the library, the user who previously used it needs to log off, so the library is no longer in use. You cannot use "fast user switching", when you are sharing the system Photos Library.

You may want to consider to share a separate library in a separate partition, a joint archive of curated, shared photos, that is not your system photos library and keep your system photo libraries private in your accounts. That will make it less complicated.

The best way to share a system library from two accounts is to store it on an external drive. You both set the same library as your system library. The disadvantage is that you must have the external storage connected whenever you want to use photos. (in fact you should have it connected whenever you log in)

The external storage must be formatted APFS or MacOS extended (journaled), and not be (or have been) used for time machine. It must have the 'ignore ownership" checkbox ticked. It should not be on a memory stick, SD or similar. It stands to reason that if you want any sort of reasonable performance it should be a fast drive with a fast connection.

It must also not be a network drive (a direct connection is required - eg USB or Thunderbolt)

See

https://support.apple.com/en-gb/HT201517

If you instead just want to move it to your wifes account, you can just copy it from one account pictures folder to the other. The safest way to do that is to copy it via an external drive, set up as described above - to avoid any permissions issues.