That's quite strange, wonder why the flight number is in the 3,000's on that route? I see from Experflyer that it's a Monday only flight and the other days it operates, it uses a normal flight number (SA 172).
The reason that the Monday flight is numbered 3172 is due to the bilateral agreement between SA and Malawi. It used to be that only 7 weekly flights were permitted between Malawi and SA by each country's designated airline. These were operated by SAA as SA 172/173 to Blantyre on Wed/Sat and SA 170/171 to Lilongwe on Mon/Tue/Thu/Fri/Sun.
When Air Malawi shut down a few years ago, there was a shortage of capacity in the market so SAA was permitted to add an additional service to Blantyre on Mondays. Since this was extra-bilateral, the service was designated as SA 3172 rather than the SA 172 that the legacy services were flown under.
Now that Malawian Airlines is up and running, they are also flying reciprocal extra-bilateral services (9x weekly flights to a mix of BLZ/LLW), so the SA 3172 flight has effectively become a permanent fixture for around 2 years now.
I thought that SA still extended *G benefits to SA Express and Airlink flights that are connected to a SA flight (AUH-JNB in her case)?
She was told by the lounge staff and lounge supervisor that only Voyager Gold (and not *G) would be permitted to access the lounge on SA Express operated flights and that she should speak to SA Express about lounge access. The SA Express staff told her (correctly) that the flight was not operated by them, so they couldn't do anything. This went on for almost 45 minutes by which time the flight was boarding so she had to proceed to the gate anyway.