It depends on what they want to do with it, and whether they have any sort of IT support.
If they have somebody doing tech support already, just wipe the drive and give them the computer (with restore discs just in case they want to use the bundled OS.)
If they don't have support but have any sort of established standard OS, then install the same OS as they're already using.
If they have no support and no standards, it's pretty much just up to you. A Linux distro if they just want to surf the web and use a word processor. Windows if they want to be able to install software purchased in a big box store.