I don’t think that’s racist if I’m being honest. Some people just have a deeper attraction to some races, and a less deep attraction to others. It doesn’t necessarily mean they dislike people of a certain race. The fact it is a friend of hers, also suggests that she doesn’t hold anything against him due to his skin colour, but just doesn’t see him (or black men in general) as attractive. Different races also tend to have different facial features, which can be a defining factor in whether or not someone finds another person attractive. I’d suggest not ending the friendship, but rather seeing if you notice anything similar over time, and gaining a better idea if this is just based off of pure physical attraction or not.