The domestic and international areas at CPT are housed in one huge terminal but are strictly separated with no way to access one from the other.

After collecting baggage, pax from either area are eventually funnelled towards a central common atrium, which has eateries and various shops, before exiting the building. This would be the logical place to meet up as you could have a cuppa or just browse a newsstand while you wait..

On departure both sections check in in the same area (one floor above the arrivals atrium) and then there are separate entrances to security that separate domestic and international passengers. So there is no way that your partner can join you in the lounge.

All international arrivals into ZA clear customs and immigration at their first port of entry - so JNB in your partner's case. Even if bags are checked through she will have to retrieve them and re-check them in JNB. So she will arrive in CPT on the domestic side.