There were a lot of political prisoners with Mandela on Robben Island. It was an iconic, memorable place, and Mandela's name would have been mentioned extensively. If it was someone in the ANC or IFP cadre, there would have been funeral processions and toyi-toyi.
I did a cursory google to figure out who it might have been and didn't get any hits, but need to try again later. It might be in Mandela's biography.