Thanks E.
It is a Paternal issue, the testers Paternal Grandmother’s line is OK, 6 DNA matches in their tree.
It is the Paternal Grandfather’s line where all the 75+ cM matches all look promising and as usual we have the big BUT, none have useful trees and none have been on Ancestry in the last year. Social Media is so far not sowing any likely candidates, well there is one but they have not posted since 2011.
10 DNA matches who share 50+cM are also not helpful so far.
It looks like successive unofficial adoptions have taken place with the Grandfather and Great Grandfather so at present determining the surname is a WIP.
Another issue is surnames, they are all over the place with Belgian, Dutch and Germanic spelling of the surnames.
South African records are pretty poor but familysearch does have a few possibles.
The tester has to tread carefully obtaining information from their paternal family so it may take time to gather the family stories.