The duchess has a close bond with her brother-in-law and has informed her husband Prince William that they will stand by the young royal, according to Look magazine.
Prince Harry was photographed naked during a game of 'strip billiards' in his Las Vegas hotel suite.
A source said: "William called Harry straight away and gave him a bit of a telling off. But Kate pointed out he had already been through the ringer and deserved their sympathy.
"She's promised to support him as much as possible and offered to help him come up with a recovery plan to get his public image back on track."
The insider said that although Harry expects some further mocking, he is putting his energy into getting back to work as an army helicopter pilot.
They added: "I think he'll be glad to be around his army mates, although they're bound to give him a ribbing over his antics, they'll also understand that getting naked and larking about is just laddish behaviour."