Kate couldn't help but offer her opinion, the US publication reports.
'She told Angie she should take Brad back,' says the insider.
'Kate said she saw what a fantastic couple they were and if Angie could find it in her heart to forgive him, it's worth giving their union another shot.
'After all she and Prince William had their own breakup and make-up.'
Since filing for divorce from Brad, 54, in 2016, and embarking in a bitter custody battle over their kids - Maddox, 16, Pax, 14, Zahara, 13, Shiloh, 12, and twins Knox and Vivienne, 10 - the former couple have not been on good terms.
Sources tell OK! that Angelina was deeply affected by Kate's advice.
'If anyone else had said such things, Angie might have been resentful,' a source tells.
'But Kate has a soft and persuasive way about her, and she was genuinely touched.
'Kate has so much respect for Angie and Brad, and wants to do what she can to help this couple in distress.'
Neither Kate or Angelina have responded to OK!'s report.