Are you implying that parent should be charging his or her parents for the work, or that he or she should cover the difference out of pocket?
And I'd recommend to anyone doing tech support for parents that they carefully guide them into purchasing a decent, maintainable machine. It's not up to me to tell OP how that gets paid for. (I've ended up buying my dad a computer to avoid him picking up a 'bargain' machine from the supermarket, just because it'd be me that ends up fixing/reinstalling that crap and recovering data...)