There's a difference between saying racist things and actually being racist. For example, if OPs friend met his mixed GF before she was with him I bet she wouldn't have any issues with her. Or if OP had taken all his previous girlfriends (who were black) out on vacations she would have no reason to believe that there is any favoritism going on her due to race.

The opposite happens with white guys. I see it all the time. Some white chick dates a black guy and they go 'she's tainted'. Why? Because they've pictured a 12inch mandingo dick in their girl. So they lash out with racist remarks. If that white girl dated a Asian guy or indian guy the white guys wouldn't give a shit because they'd perceive them as lesser (even though they could be hung like a stallion). It's all just perception.

Racism is a lot deeper than just wanting to hurt someone's feelings with words. It's actually believing that race is lesser than you. OPs best friend is clearly intimidated by his girlfriend. Not condoning any of this but I feel like people throw the word around way to much.