I would have made attempts to include my friend (and this is regardless of race) into a conversation where the other conversationalists were not including her. I am not sure I could be relied upon to know for sure that the white couple was excluding her because she was black. I might wonder that, but I wouldn’t immediately think, “Oh, this couple is racist.” I’d probably say things to them like, “Oh, you’ve been to Nepal? Naomi here just went there last year!” or whatever where relevant. If they didn’t turn to her and start asking her about Nepal I would think they were racist.
I also notice if I go somewhere with a black friend and s/he is the only black person there or, worse, the only POC who is not one of the staff. I would feel uncomfortable in those environments by myself, and intensely uncomfortable if I were with a friend. I’m not sure what I would do. I might whisper a joke to him/her about it or ask if s/he wanted to leave.
It’s a long process becoming aware of all white environments and how I was raised to take them for granted. It’s a lot more striking now having lived in New York for 20 years. The all white crowds I see when I travel to different parts of the country feel weird to me now.