Any question answered...

A fashion photographer of mine has been getting really annoyed with his models re-photoshping his work and showcase their bad photoshopped results... he usually sends them a bunch of fair quality jpegs so they could have for their portfolios, nothing poster size but good enough for the models to mess with the files. So we want to know if there is a way to lock the jpeg files? We have tried saving them as PDFs and lock it that way but Photoshop can just override it. Simply locking the actual files to read only isn't good enough as it takes a 5 years old to work out how to untick the box. Any suggestion?

Watermark and DPI. Save sample files in low DPI so any work they do on the photos will almost always make things worse.
Why not just send them samples of already Photoshopped work? (If feasible) That way, any further work they do on it will be glaringly obvious.