There are a few problems with your friend's assumptions. First and foremost, there were about 12 million Africans loaded onto slave ships during the nearly 400 years of the trans-Atlantic slave trade, so any story about all slaves descending from African royalty is self-evidently nonsensical. Secondly, "civil war" is really not the right word to use. There was a great deal of fighting among the various kingdoms and peoples of Africa, some (much) of it encouraged by the fact that the winners could sell the losers for great profit. Some fighting did occur within African polities, destablizing them. This is what happened in the Kingdom of Kongo, which prompted King Nzinga to write to the King of Portugal. One of Nzinga's complaints is that slavers had captured and sold members of the royal family, which they were not supposed to do (commoners were fine to enslave, of course). This is probably what your friend was thinking about, but again, it is one example from a trade involving millions of people over hundreds of years.

The most comprehensive resource for the trans-Atlantic slave trade may be this website. Among it's many useful parts is tracking which slave ships went where, which has shed new light on how we think of the African influence on the New World.

Thank you all for your answers. She said that her history teacher taught her that all black Americans whose ancestors were slaves, were actually of royal blood. That they were all kings and queens. I didn't question her because I don't know that much about it. When she was in school, the schools were still largely segregated. I am much younger than she is and I'm white. I understand that history textbooks can be biased and I wasn't sure if her teacher was being overly biased or my textbook just "overlooked" that detail.