Unfortunately the only photo library I can find on the computer is the 'Photos Library', which is empty 😟
Is the size of this library as shown in the Finder small, like for an empty library or is it large, but showing no Photos?
How did your mother do the upgrade to El Capitan? Did she erase the Mac and make a clean install or install on top of the old system?
Try to find the original iPhoto library. Perhaps it has been moved or corrupted. Search with SpotLight or FindAnyFile for traces of the iPhoto Library.
Download Find Any File here: The trial is free:
http://apps.tempel.org/FindAnyFile/index.htmlhttp://apps.tempel.org/FindAnyFile/index.php
You can search everywhere with FindAnyFile, where Spotlight will not search.
I learned this from Old Toad , see his post here:
Re: My iPhoto Library disappeared...how do I get it back?
Download and run Find Any File to search for a file with "ThumbJPGSegment" in the file name. These are files common to all versions of iPhoto libraries.
FAF can search areas that Spotlight can't like invisible folders, system folders and packages. If there's a library hiding somewhere on your hard drive FAF will find it.
If you cannot find another library, try to restore your iPhoto Library from the last backup you made.