He'll have to ask the card co what happened. African card holders buy online things every day they don't get blocked so his domain purchase shouldn't have been rejected. He should ask the card co why they didn't let him buy the domain.
He didn't give detailed explanations on the process he took trying to buy the domain and the kind of card he used or which provider he was dealing with, but we get the message he was trying to pass. Bitcoin extended the opportunity of doing business across international boarders without interference from government or requesting any layer of verification causing limitations. For any cross country deal with Bitcoin payment option, it is considered the best selective option.
It doesn't even matter about this particular case the general idea is what is more important. Many people don't realize that this happens all the time and every day for many people in the world. Sometimes even temporary outages can block you from important services. Other times it is technical bugs with compatibility of merchant implementations. With Bitcoin it is much easier, an address and an amount are generated and you send it from wherever you want. There need not be special implementations relating to merchants, banks and processing companies. Further discriminating users in advance is never a good thing.