Jennifer Aniston tells Katy Perry: Don't marry John Mayer!
'Jen will never get past what John did to her,' says a source.
'When they dated she really thought he was "the one", but it was horrific when he dumped herby 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.'
Jen, 44 - now engaged to Justin Theroux, 41 - dated notorious love rat John for 11 months before their acrimonious split in March 2009.
She's stressed to friends that she's far from jealous, saying she doesn't want to see the former Mrs Russell Brand get hurt again.
However, John, 35 - who previously romanced Jessica Simpson and Taylor Swift - has insisted to pals that Katy, who he's been dating since last July could be the one to 'tame' him.
Read more about Katy Perry and John Mayer in Now magazine dated 18 February 2013 - out now!
SEE PICTURES Russell Brand and Katy Perry - a love story in pictures
SEE PICTURES Russell Brand and Katy Perry in celebrity splits 2011
SEE PICTURES Katy Perry's ex-husband Russell Brand at Rock Of Ages premiere in London