Pretoria is a lovely city, and with the resources given to diplomats your brother should have no safety issues. That said, there are a number of things that may come as a bit of a shock. I'm not sure which country you're from, but many expats in Pretoria are surprised by the heavy amount of security in the form of armed guards, gated communities, and the high security fences with barbed wire and electricity.

Here's a good general guide to safety in South Africa, and here is a link to our Pretoria Travel guide, which has a lot more great information on Pretoria.