In my experience (from the handful of bookings that have come to attention for some reason or other at London) COMAIR handles this exactly the same as London.

The call centres will tell you that ticket is now null and void, unable to be used and normal refund conditions will apply.

At the airport, they wil tell you also the return has been cancelled, the difference being they will reinstate the booking in whatever booking class available/applicable, reprice the ticket as a one way, exchanging the ticket you already have and using the value of that against the one way journey but you’ll likely also pay the change fee/no show fee on the original ticket as in theory they are now changing that.. So depending on the fare you paid, it may or may not be worthwhile.

If its 161 ticket stock, they may handle it differently as this will be under kulula rules and I don’t know how flexible they are and their bookings/ticket stock would not come to our attention in London.