I have found a photo of Johannes Christoffel Heyneke. It appears that he ewas in the Smuts commando and at Modderfontein on the 17th September 1901. (A Tight Corner by Simpson & Fahler). He was wounded somewhere and ended the war as POW on St Helena.
I'm trying to find details about him besides that he was a member of the Theron's Scouts or TVK.
Where was he wounded and taken prisoner??
To help with your searching I have cross referenced some of this information from the website GenDatabase.
Boer War prisoner record for: Johannes Christoffel Heyneke Age : 26 Address : Pretoria
Arrest Information Place Arrested : Tienfontein Date Arrested : 1901-11-11
Detention Information Camp Detained : Country Detained : St. Helena Prisoner No : 27030
From the information that Rory was able provide and using the correct spelling of your Commando and the place of arrest as Tienfontein, then I'm quite happy that this is the information that you have been chasing -
The other information that you provided with regards to his death aged in his 50's in 1925 would also come close to making this Commando as your person of interest.