Is the word racist, as capriciously applied to white people, a slur?

My son’s girlfriend is/was Hispanic. And illegal, maybe relevant maybe not.

In my presence, she called him a racist because he voted for Romney.
He is 21. Naïve to be sure. But, the accusation upset him.
I told him that it should indeed upset him. I told him it was slur since there was no evidence it was true, other than he voted for somebody she did not like.
He dumped her.
I thought it was a good move. I think it is indeed a slur.

Or not?