Bad news, I'm afraid. Square 79 is in the the heavily overgrown part of the cemetery in and around the figure-of-8 paths on the right on the map you've been directed to on Find a Grave. It's in the lower right corner of that area, well off the nearest path and unlikely to be possible to get to the grave safely, if at all. I have seen detailed grave-by-grave maps for some squares, presumably supplied by the office (at Camberwell New Cemetery) but I doubt that they'd be of any use in this instance.
For anyone else coming across this thread, I'll mention that the aforementioned map is out of date as there has been relandscaping and re-use of squares 1, 6 & 7 and thereabouts around the side gate on Wood Vale and 21 & 100-104 off Underhill Road. It's unlikely that any old gravestones in these areas can be found, and many elsewhere throughout the cemetery have also been removed.
The above situation is why there are well over 2000 photo requests on Find a Grave, most unlikely to ever be fulfilled.
Having given all those gloomy tidings, there is now a cemetery friends' organisation which may be better placed to help and advise than me, though do feel free to ask, especially before travelling any distance.