Katy Perry and Rihanna's friendship is reportedly on the rocks because of Chris Brown. According to Perez Hilton, Katy was extremely unhappy that Rihanna kissed Chris Brown at the VMAs. Rihanna then reportedly bailed on Katy's birthday party -- while Katy did not attend Rihanna's Halloween celebration.  A source said, "They're barely talking and ever since the couple reunited, there has been tension between them. Katy has been spending a lot of time with pals Robert Pattinson and Kristen Stewart. Katy fears Rihanna is making a huge mistake and doesn't want to be part of it."