You have no idea when in her genetic history the mixing occurred and if it was consensual or not. Or maybe you do but I haven’t seen the timeline. Entirely possible it happened later than the slave era and was consensual.
I remember an interview on NPR about how ancestry DNA type services used mitochondrial DNA in the African American community, especially for males. Seems there are a lot of European Y DNA matches in the African American community.
b’99 is right about the common-ness of it during slave times.
Z is right about assumptions.
Doesn’t change the fact that this-person or that-person is someone’s ancestor.
In fact, if the game here is to suggest that the slave-owning sins of your ancestor somehow apply to you, then ancestry via rape means you have the DNA of a rapist in your as well.
Maybe it’s time to stop the blood-guilt exercise and see today’s person as his own self and therefore can only be responsible for his own actions.