Turns out there was nearly 10GB in videos, so I selected all of the items in the Movies album and exported them to an external disk. After removing these from the library, emptying the iPhoto trash and the actual Trash the disk had about 15GB free.
Not so fast
Of course not. After handing the laptop back my mom complained that iPhoto was running really sluggish. I got it back, checked it out and it was unusable. Scrolling had a 5-6 second lag and the UI responsiveness was generally pretty terrible.
My guess is that removing those videos screwed up the library somehow, so I opened up iPhoto holding down CMD+OPT to get the repair menu up and ran each item on the list (repairing permissions, rebuilding thumbnails, reindexing photos, repairing the library). Many, many hours later everything was still terrible so I decided to start from scratch.
My mom's iPhoto library is fairly large — nearly 15,000 photos. Initially I thought her 11" may not be able to handle that many photos so I figured exporting all of the photos and only importing the ones from the last two years would be a way of seeing if that was the case.
I left the laptop overnight exporting the 15,000 photos, backed up the library and deleted the original. Now I just had to find the photos taken within the last two years.
What should be easy...
Once again foiled. All of the exported photos had date of creation and modification set to yesterday. The sub-folders were all album names and not really indicative of when the photos were taken. I peeked inside the iPhoto library package contents to the Original and Modified folders where the photos live but found duplicates, terrible names and general cruft so I figured the actual exported photos had to be salvaged somehow.
Then I remembered EXIF data. Every photo you take contains metadata about what camera was used to take it, the model, ISO, exposure time, etc. One of those fields is the date it was taken. So if I could figure out how to read the date taken I could write a script to "fix" the photos.
ExifTool to the rescue!
With ExifTool in hand I went off to Automator and wrote some Ruby that would loop through a set of folders and change the date modified to the date the photo was taken as dictated by its EXIF data. It looked like so (download it here):