I suppose if you friends are authorised by you this might be possible. But they will need to have the carnet document to present this to the customs officer, which will be filled out and stamped there.
Again, crossing a border will require an authorisation that this person is moving your vehicle whith your knowledge and permission. It would be best to clear this with the S.A. embassy in the UK. Additionally you could try to get some info from the South African AA. See their web site: Automobile Association
Finally - according to the German AA, the ADAC, it is only possible to extend the carnet for a second year. Cross-check this with your local RAC. Besides that, not all countries need a carnet. Your RAC should have information on this issue available.
Only when we pause to wonder
do we go beyond the limits of our little lives.