As BA2065 was descending into MRU, I went to the Air Mauritius check-in desk, explained the situation (MK flight on Saturday MRU-JNB with Chinese passport, no SA visa wife connecting to LHR later same day on BA), and sought confirmation that she would be allowed to board. "Yes of course", with an animated delivery.
Good to hear your family have made it. I'd feel the same as your wife.
When you spoke to the Air Mauritius staff, did you check the return had not been cancelled because your family did not take the outbound?