Yes, definitely, too big, are you mad, never, need a 1300, 4 rubbers...
Ask 10 people and you'll get 20 opinions. To give an accurate answer we need to know where he would do most of the diving as we can give an idea of what species he can encounter. But generally for SA conditions that gun is not suitable. If it's 500g Blacktail, yes (but good luck trying to shoot a Blacktail with a 500, not ever going to get close enough)
Basically all he is going to get would be the odd doff Karanteen and really small stuff which wouldn't even be a mouthfull. Those 500 guns are very useful for e.g. Conger eels in caves in 1m vis. In SA, you're never really going to dive in those conditions.
Depending on where your mate will dive I would say a 900-1000 and can get fish like Shad (Elf) and even Garrick when they're around. These require no "skill" in shooting but with Garrick, landing them is where the fun is. Plus in and around the bricks you can swim into a 40kg Kob, Cracker etc. And garaunteed, when he sees his first big fish and gets more experienced, he will upgrade to give greater flexibility and options in good conditions.
Hope that helps.