A nanny visa?

If a South African were to come over here to be a nanny, what would be the visa process they would have to take? My friend asked me to about this information, and I had no clue. Can they come first and then apply for a work visa of some sort, or do they have to have an invitation from my friend? If the second, what office would they have to contact to do this? Thank you for any information that will help.