Lubuntu fits in ~1GB? Remember that whatever he installs on it, there needs to be lots of free space for other data.
Last time I did something like this (with a 2GB SD card - yeah, emergency OS) I had used some distro that would let you choose the packages from the site and it'd generate a bootable image for you ready to copy in the drive, but I can't remember the name right now. There's also this http://www.damnsmalllinux.org/ 50MB only. Not sure how usable it is, though (or if it's maintained, I don't really track distros).