extend your current Tourist visa with 30 days, then apply before the total of your 90 days expire (and before March 26) for 60 COVID extension days. Easypiesy since you are a new tourist, no problem
If she booked 2 separate tickets hence South Africa-BKK and needs to check in for BKK-Cambodia then she needs to go through Immigration and do all requirements for entering Thailand.
Best firstly to check with booking agent or airline, plus Cambodian requirements for entering on sealed air-routes.
If she needs to go through Immigration to pick up her luggage I could imagine its meaning Thai Pass, Test & Go scheme (which is 6 days stay in Thailand but maybe 1 day Quarantaine hotel in this case??), PCRs tests, insurance etc
She most likely needs a sealed route to fly to Cambodia, might be cheaper and less time consuming to cancel and book another flight.