The fact that you have a friend of X race does not fully tell how you treat other people from his race.
As a better example, a man can love and respect his mother r. However he can have a misogynist behaviour towards all other woman around him. He cannot justify that he is not a misogynist just because he treats his mother with respect.
Also, “friendship “ can have many meanings, levels or reasons. An office friend is not the same as a life long friend in general. You can be a friend of a single person of X race for a specific reason, or even just because you got used to his presence. But it might not be a “full” friendship from your heart. He could be more like a colleague for example. Or you think you are his friend because you treat him nice and help him, but you could be seeing him with a sense of superiority (you think you are better than him, so try to help him with some pity), and this could be because of race.
It could be even unconscious.

I might not be explaining much. But hope i give you some examples of why having a friend from X race is not a logical proof to say you are not racist.