How can I merge two photo libraries on Mac?

Does the wife have access to the iCloud account of her husband at all? As a legacy contact she will get access to a new account, where the iCloud Photos will be stored, see: How to add a Legacy Contact for your Apple Account – Apple Support (UK)

The easiest way to merge two iCloud Photos Libraries from two different Apple accounts would be to set up a Shared iCloud Photos Library. How to use iCloud Shared Photo Library - Apple Support

This would make it possible to move all photos from the husband's library to the Shared library and they would appear in the wife's library in the shared section. The items would be transferred as a pair of the edited version and the original, in the full quality, with all metadata. It would be a lossless migration.

Only the albums and folders would be missing.

To save the albums as well, you would first have to turn the husband's iCloud Photos Library into a non iCloud Photos Library, with all original image files downloaded, so the library is complete. Then it should be copied to a folder, where the wife as read-read permission. Now the wife could enable iCloud Photos for this library and iCloud Photos will merge it into the wife's iCloud Photos Library, preserving the folders and albums as well. This will take a lot of time, however.

If the libraries are both the same version you can merge them with the paid version of PowerPhotos. The current version of Photos can only import photos from another library and only the original I believe. PowerPhotos can merge photos (originals and edited versions), albums, smart albums as albums and projects as albums. Here's a comparison between the two:

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

Smart Albums (as regular albums)** -----------

Folders Containing Albums -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.

Things depends on what version of Photos you (or she) have. Current versions will not merge but will allow you to import from one Library to another. Very reliable and worth the investment PowerPhotos:

https://www.fatcatsoftware.com/powerphotos/

It is probably the most simple and uncomplicated to way to achieve what you need.

Good Luck!