Though their friendship was one for the vocally overprocessed, musically overproduced and visually overdone ages, Katy Perry and Rihanna are dunzo.

And guess who's behind it?

Perry is reportedly furious that Rihanna is back with Chris Brown, who, you know, savagely beat the snot out of her before the 2009 Grammys.

"Katy was always the one person who could really stand up to Rihanna," a source told Grazia (via The Telegraph).

News of a tiff between the two first broke in November, and it's only gotten worse since. In fact, the two gals who almost always sit together at awards shows were pretty much miles apart all night at the Grammys last night (Feb. 10), where Rihanna cozied up to Brown.

"When it comes to Chris, Rihanna isn't willing to listen. Katy is still incredibly angry about what Chris did, and she doesn't believe he can change."

Indeed, Brown's brawls with Drake and then Frank Ocean don't negate Perry's point at all. (Seriously bro, who's next? A kitten?)

"She has stuck her neck out for Rihanna, and so it really hurt when Rihanna ignored her advice," the source added.

"Katy told Rihanna that she couldn't stick around and watch someone she loves make such a big mistake, and that she had to walk away. They haven't spoken for months. Chris is like Rihanna's addiction and no one can talk any sense into her."

Of course, this may seem somewhat hypocritical coming from the woman who married Russell Brand and is currently dating John Mayer, but those guys never physically hurt any of the women they were with. Well, no harm that couldn't be cured with penicillin anyway.

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