That you mention Stellenbosch brings up two thoughts in my head. Either (a) they're doing something at Stellenbosch University or (b) something associated with a winery.

The only things I could think of would be trying to find out if there's someone (either a student or professor) at Stellenbosch University looking to 'rent' their car for that time frame. If I'm guessing right, maybe your friend has pre-trip access to some community / bulletin board of folks at the University or a particular program where s/he can ask around or get advice.

In my experience, some wineries bring in kids (mostly) (OK, young adults, we're not talking child labour) from abroad who are doing some sort of viticultural type degree. During harvest, the wineries work them like dogs (though that's assuming dogs are worked to death) and put them up somewhere nearby. Often, there's a knock around "winery" car or two they have access to during their stay. Maybe that's something they could ask about.

I'm pretty frequently in and around Stellenbosch and can't, off the top of my head, think of anyplace other than the major car hire companies (Hertz, Avis, definitely, Sixit, I think) that I've seen around. There are places that sell used cars (Johan Behm - a repair shop - is where a friend sold one or two of hers for about ZAR20,000 each - they were old and out of style but very reliable cars). Try them as they may have good old cheap car that they could buy and resell at the end of their stay. EUR 500 is pretty low though.