EDIT Title should say West African. Maps are hard. Also assorted typos.
One of the slavery apologist arguments I heard from a friend of mine is that "The Atlantic slave trade wasn't really breaking up families, because Africans back then didn't have any concept of marriage; whoever you were sleeping with at the time was your husband(s) or wife/wives".
I'm normally pretty good at refuting the usual apologist arguments but this is something I haven't been able to find reliable information on, even though it reeks of BS; even if they don't have an organized religion or one man/one woman lifetime commitments most cultures have developed a concept like marriage, even as simple as "ownership" of a woman/group of women. I understand if this is better asked on r/askanthropologists but I'm trying here first.
Since you mention that this question was generated by discussion of the Atlantic slave trade, did you really mean East African (i.e. Indian Ocean) families, or did you mean West African (i.e. Atlantic)?
Whatever the case, your friend is completely full of shit. That's like 10 pounds of shit in a 5 pound bag levels.
Of course there were concepts of marriage!! Marriage and kinship systems has been something ethnologists and antropologists have long had an interest in. I have a book on my shelf right now, published in 1950, titled African Systems of Kinship and Marriage, which uses some dated language but it's central purpose is to describe different societies across the continent, catalog whether they practice matrilineal or patrilineal or double descent, and what terms they use to refer to aunts, uncles, cousins, nieces/nephews, etc.
In Paths in the Rainforest, Jan Vansina has provided a reconstruction of changes in societies of the Central African rainforest from earliest times to the 1880s. A central theme of the book is how important marriage and kinship ties were in generating more complex and more extensive chiefdoms and kingdoms. His more recent book How Societies Are Born also talks about this.
Thomas Reefe in the Rainbow and the Kings and David Birmingham in Central Africa to 1870 also talk about the role of "fictive kinship" in Mbundu, Luba and Lunda societies. That is, a Lunda noble would interact with other nobles of the same "rank" as him as though they were brothers and would address them as brother. He would interact with more powerful nobles as though they were uncles. He would interact with a subordinate as though they were a son or nephew.
Sorry I don't have more time to answer more fully. Suffice to say, Africans absolutely had a concept of family, and they knew who they were related to, and the Atlantic slave trade did break up families.