A person who flees to the United Kingdom to seek asylum can include his/her dependants in his/her application for asylum, if those dependants have travelled with him/her to the United Kingdom. However, the UK Border Agency recognises that families can become fragmented in cases of asylum, depending on the speed and manner in which the person has fled.
If the person is a recognised refugee or is benefiting from humanitarian protection in the United Kingdom, the UKBA's family programme reunion allows him/her to be reunited with his/her family members (that is, those who were part of the family unit before he/she fled).
Under the Immigration Rules, only pre-existing families (the spouse, civil partner or unmarried/same-sex partner plus any children under 18 who formed part of the family unit at the time the sponsor fled to seek asylum) can apply to enter the United Kingdom under the family reunion programme.
I am assuming that your fiance did not have a family before departing South Africa, therefore, the family reunion programme will not be applicable.
How long has your fiance been in the UK?