Create child theme from modified parent theme

Hey,

I’ve been given the joyful task of getting a site that a 3rd party developed from a themeforest theme through W3C validation. Unfortunately, that 3rd party over-wrote the parent theme styling etc instead of setting up as a child theme, so I can’t run the theme update that addresses a few of the W3C issues (among other things). Does anyone know a quick and easy way of isolating the changes from the files (short of individual side by side filecomparison?)

Thanks,

Dave