Your example of a naturalized Caucasian US citizen from S. Africa is one that has cropped up closely in my life.

My youngest brother was born in what was then Ethiopia.  Now it would be Eritrea.  So, he was AA, as your example indicates. Though we have the same very Caucasian parents.  

I was raised internationally, and have spent significant time living, working, and traveling in Africa.  

I once shared a cubicle with a black woman. We were both new hire engineers working for our first company after completing Undergrad, in the US.  We got along great. I noticed a wedding picture on a shelf on her side of our two-station cubicle.  I asked about the picture. We engaged in a conversation on her wedding. She and her groom wore "African Royal" wedding attire. I asked where in Africa.  That.... was apparently a big mistake on my part.  The following conversation ended up with an HR complaint against me.  I asked a few questions on where in Africa the wedding ensembles were from.  The only answer I got was that they were used by African royalty.  After the conversation apparently, she decided I had insulted her African ancestry and filed a complaint. She had no idea what her ancestry was. Only that it was African.  Our conversation included that I shared that my youngest brother was born in Africa. My coworker had taken that fact, did not ask any questions, and decided my parents had adopted an African baby and denied it of its African culture. Nothing came of the HR investigation. I was counseled to not discuss the topic with her again.  It was an extremely uncomfortable and enlightening experience.

Something that I have gained clarity on since then is that she likely did not know any specifics regarding her African heritage. That did not dawn on me at the time.  My own experience fed a bias. I assumed that she would know where in Africa her wedding attire was from.

That was my introduction onto the thin ice that white people may be walking when conversation on Africa occurs in the US.

I am fortunate to have traveled extensively in Africa.  I have worked with incredible professionals from across Africa.  They are all Africans.  Though interestingly, they align with their country of origin. Not their continent.  It seems that US Americans are far more likely to categorize their background as African.  The actual Africans I know align as Kenyan, South African, Moroccan, Egyptian, Ethiopian, Kenyan, Tanzanian, Nigerian, Senegalese, Sudanese, etc....  This is not surprising considering that many black Americans of African origin do not know the details of their African background due to the loss of that knowledge. One of the many tragedies related to our nation’s history of slavery.

We have close friends who are South African. He is of British extraction and Caucasian.  She is of Indian (subcontinent) extraction and Asian. They are proudly South African and will tell you they are African.  Your observation on this caveat is a very important one. I wish it was a more common point of knowledge for US Americans.

I have also experienced being one of the few "minorities" in a room in business environments.  Depending on what continent I was working on at the time. Being cognizant of who is present is an important observation to make. A humorous incident that occurred when I first returned to the Middle Eash for work.  I understand a bit of Arabic. I was in a meeting where side bar conversations were happening in Arabic. I had a boss who was a pompous asshat. The side bar conversation was highlighting that fact regarding my boss who was also in the meeting.  I reacted to their side bar with a disguised cough and a grin.  After that, at the beginning of every meeting someone would comment that I understood and there would be no side bar conversations.  The entire meeting would be conducted in English.  Having been both a minority in professional meetings/work environments, and in the majority, I try to consider others in any meeting I am in.  What simplifies that is to consider participants by their role not their continental background.  Understanding cultural influences is important.  Balancing between the role someone fills and what are the likely influences they bring is a beneficial skill.

Everyone has bias of some form or another. I have seen it in Europe, North America, Africa, Asia, the Middle East.  While I know I have bias as well, I attempt to alleviate it by engaging people as individuals rather than as a member of a fabricated categorization.  I engage engineers as engineers, accountants as accountants, leaders as leaders, etc.....  Their melanin levels are irrelevant in that. Though we certainly all bring our backgrounds to any interface. Background is important, color is not.

IMHO and experience anyway.

Thanks for sharing your perspective.

Everyone apologized profusely and over the course of several days for the offense - the ramblings of an elderly Portuguese woman. I know people should choose their words carefully so as not to offend, but English is not the grandma's first language. I wonder if this whole thread is some kind of social experiment to see how far people will go to prove they aren't racist. Disrespecting your elderly mother - check! Putting up with the emotional abuse of your stepdaughter - check! I want to clarify, racism is NEVER ok. But, this OP has wracked her brain to try and atone for the brunch comment as well as to try and figure out if she or anyone could have possibly, UNINTENTIONALLY, made some sort of comment in the past that may have triggered this young man's reaction. Screaming at SD, banging his fist, leaving, coming back, shunning her though she cried for DAYS, telling her to do some reading but when she begs to know what she should read -more emotional blackmail. Enough is enough. I really hope the outcome isn't this young woman's suicide. ETA OP, i hope that's not the case and i'm not blaming you if this is real and that is what happened. From what you've said, you have bent over backwards to try to do what you believe is the right thing for all involved.