The construction is far from over! But the good news is that you do not need to go outside any more: the international arrivals now disgorge into an internal plaza directly linked with the domestic terminal. (You will still have to collect your luggage, though.)

For meeting friends my preference is the ground floor lobby lounge of the InterContinental. (Exit terminal building directly from arrivals, cross the road and keep going and within 50m, hey presto, there it is on the right.) It is usually pretty calm and quiet and they have decent coffee. This is where I schedule any meetings while I'm at JNB.

Alternatively on the mezzanine level of the domestic departures terminal there are restaurants/pubs/coffee shops that may be suitable. I have never even walked past them so, sorry, I can't make a recommendation. Likewise you could backtrack to the international terminal but again I can't comment on what's on offer.

Going straight ot the domestic terminal you could check in and get rid of your checked luggage. But it is a very short walk back to the IC and it's much more salubrious than the airport cafes I would think.