Just a tip to pass on to your friend, it is better not to try to compress photo files for two reasons:
a) As Dave points out they are already in a compressed format.
b) Where possible (and time permits) any photo backup should be checked for corruption caused by the copying process.
I do not have independent stats on file corruption but corrupt headers are fairly common. I’d guess that it could be at least 1:1000 that are corrupt in some way. From bitter experience, that number sky-rockets if you compress the jpg.
JPG Checker is a checker for Windows. (not now free). I have used it in the past when it was free and it was fast and accurate.
If he knows how to use Python then PIL is a good method as it actually loads and tests each image and prints out an exception for those that fail. Unfortunately this really only detects corrupt headers, not image glitches.
Python code, (needs to be put in an appropriate loop):
from PIL import Image
v_image = Image.open(file)
v_image.verify()
If he knows Linux then jpginfo is a free way method.
This is not an exhaustive list of methods, but rather to encourage your friend to do it if the photos are at all valuable. He can find more by searching on-line for “check jpg files for corruption”
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