How was he an observer? And when? A commando was a temporary formed group from civilians as needed. So was he living with Joubert watching him farm? More likely he was not part of the invading forces. Cam Simpson may know more.
I see from a post you made 5 years ago that you were compiling a list of raiders' medals that were extant. Not sure whether you are still interested, but I have my grandfather's BSAC Medal. It was for the Matabeleland campaign in 1896 and he was in the MRF. For the Jameson Raid he was in D Troop.
To add to the list, I am the current custodian of the medal to 1283 Hosp Sjt Maj HJ Sutcliffe, BB Police. I know that Herbert John Sutcliffe was from Beverley in Yorkshire, but can find nothing on him post his return to UK after the Jameson Raid. For example, did he return to South Africa during the Boer War? Another research challenge!
If it helps your sanity, Nemo, I did not come across him name either when I looked at the passenger manifests for the south-bound voyages carrying the groups of raiders returning to put down the Matabele rebellion between 14 March and 11 April 1896 when about 180 raiders returned. However, the award of the BSAC Medal shows that he did! Is his medal for Rhodesia 1896? Which unit was he attached to? I have a feeling that most went in the MRF under Plummer.
Good hunting and if you come across any source of attestion papers, please let me know!