A decade ago my internet subscription was also capped at 10Gb per month. Back then this was already quite strict, with the whole family having an average usage of 5-7Gb per month, there wasn't much room for downloads.
I can imagine that he cannot download 5Gb of data as that would mean the whole family won't be able to properly internet for perhaps a whole month just to get a game. With Youtube and all it's probably even harder now than it was a decade ago.
I agree with previous post probably the easiest way would be to lend a USB stick and go to some place (like a friend or public internet) where you can download it and then bring it home.
CD's will be too small, they can only fit 700Mb and a rewritable DVD might be just marginally too small aswell. USB sticks have 8Gb to 32Gb capacity nowadays