Jennifer Aniston has allegedly warned Katy Perry not to marry John Mayer. A source tells NOW magazine, ''Jen will never get past what John did to her. When they dated she really thought he was "the one", but it was horrific when he dumped her by text message. Having seen how serious they're getting, Jennifer told Katy not to believe his "pretty words". She's also urged Katy not to believe...or accept...any proposal unless she's absolutely sure he's changed.''