If you apply to the consulate of South Africa before traveling to the US and provide all the documents confirming the trip to the US, you can apply for a South African visa in advance. The citizens of Russia can receive South Africa's visa only on the territory of the Russian Federation, for example, in Moscow.

The South African visa is issued at the South African Embassy in Moscow, a maximum of three months before the start of the trip. If a tourist does not have a residence permit in America, he will not be able to apply for a visa in South Africa to the United States.

You can try explaining the situation, get a visa in Moscow before departure, perhaps the tourist will meet. I advise you to call the Embassy of South Africa in Moscow.

You can use both of these options. Make a visa in Russia or come to the Consulate of South Africa in the US and make a visa there. Only it is necessary to consider that in Russia the visa of the republic of South Africa is made 2 weeks.