Moving photos from one Photos library into another
I think the easiest way to do this is to export the photos from her library and then copy them to /Users/[username]/Public/Drop Box. You can access these public folder from every other account on your computer and import the photos into the new library.
Really, you can't.
https://discussions.apple.com/docs/DOC-10211
Short answer, if you like to edit, rate, or adjust your photos. You have no way to migrate photos in libraries non-destructively without paying Apple for iCloud.
It can be done with PowerPhoto's merging capability. It's not free though.