That may be, but she could have just simply corrected him on that and it would've ended right there; instead she decided to get others involved and claimed it to be "discrimination".
Or he could've not said what he said at all and there wouldn't be an issue.
He's the one who had to bring race up.
He could've said "but you speak spanish don't you?", but instead he said "but you're mexican".
So first he assumed she was mexican based off her skin, and then assumed that she would be fluent in Spanish just because of her nationality.
It's not a horrible crime he commited. It was just a mistake.
But it's not the girls fault it happened.