If people are gonna be mad at Rih then they need to be just as mad at homegirl's friend who watched Rih do it and didn't do or say a damn thing about it.
I wouldn't say anything either. It's Rihanna. I would feel awkward and starstruck. What do you say? "Rihanna just put some popcorn on your chair." I probably would've kept quiet in that instance too.