During the unannounced appearance, she paid her respects and left flowers for the young woman at Clapham Common.
Royal biographer Andrew Morton commended the Duchess for letting her actions portray her emotions, like Diana did.
He told OK!: “She didn’t have to say a word, just turned up and laid her flowers and everyone can draw their own conclusions.
“That’s exactly how Diana did it – one gesture is worth a thousand words.
“It shows that Kate is someone who doesn’t have to speak out.”
He added: “She’s doing what Diana used to do, which is let her body language tell the story.”
According to Mr Morton, also likened both royal’s stride, adding: “Kate has the Diana Walk.
At the time, a royal source told the Mirror that Kate “wanted to pay her respects to the family and to Sarah”.
They added: “She remembers what it was like to walk around London at night before she was married to Prince William.
“There was no fanfare, this was a private matter to her and she wanted to show unity with everyone else who is having the same feelings right now.”