You could do this by creating the slideshow using a folder full of stand-in images with serial names:
project.aep
images/
01.jpg
02.jpg
03.jpg
You can save an Effect preset with an expression that does your transition based on the in-point and out-point of each slide, so then all you have to do is add the numbered images to a comp and line up the timing.
When you’re friend has her photos ready, all he/she has to do is just rename her files with the same serial numbers (you can use Bridge to do this in bulk), and then replace the stand-in files in the images folder, render, and done!