I'd say either let it go, since she got perfectly good service from a normal, decent SA. If that hadn't been the case, then take it up to managment.
From my POV, there is a difference between seeing/experiencing racist acts as a private person and making a fuss about it because you're "famous". There are different ways of dealing with such situations.
I'm sure that Oprah must have said something to someone that originated that rumour. Maybe it was a genuine mistake (a moment of anger/dissapointment), maybe it was being a drama queen but her reaction about still shopping Hermes didn't touch quite as many people as the story of racist Hermes.
Here, clearly your friend and her mom were discriminated against because ONE SA was at the very least, a total b*tch. It's sad, it happens everywhere and I would have done something about it if I had heard the SA say that.
But it may have been the way they were dressed or whatever as well.
I tend to get that a lot when I go to stores, I look young, dressed casually, SAs tend to snif at me and whatnot. It's a different story when I pull out the cash.
Also, I agree with the statement that H's CEO had to placate Oprah but that isn't because they may/may not have done something wrong, it's because the US is a big consummer and many people there actually take to heart what she says.
It's good PR, not the sign of a guilty conscience.
The real person I pity here is the poor SA who probably got fired because some talk show hostess she'd never heard about wanted in after hours and got turned away.