My advice...

I bet nowadays there are hardly any racial discrimination cases that is as obvious as black n white, but very subtle and most of the times they would fall into some gray areas...

It is very hard to have a case in the court of law unless you have some hard evidence.

I'd suggest you to video tape the incident(if you ever encounter one again) like this one and then talk to a lawyer or post video on Youtube to get more public exposure.

Tell the your girl be strong, be confident and be herself!