The loved-up couple - who recently got engaged themselves - were so impressed with the wedding of Whitney Madian and her husband Shant at the Oak Bay Beach Hotel in Victoria on Saturday, which they witnessed from their hotel balcony, that they went down stairs to secretly sign the newlyweds' guest book at the reception.
Whitney told TMZ that she saw the pair scribbling away in her book that she approached them and ended up having a conversation with them for five minutes.
The blushing bride explained that Katharine and David had told her that they watched her walk down the aisle from their balcony and thought the wedding was ''beautiful.''
The bride then compared her wedding band with Katharine's engagement ring.
The 34-year-old actress got engaged to the record producer, 68, at the beginning of last month following less than a year of dating and can't wait to become his wife.
At the end of last year, Katharine revealed she had been friends with David - the ex-husband of reality star Yolanda Hadid - for years before their relationship became romantic.
She explained: ''We're very close friends, and we've been friends for a long time. I'm really, really fond of him, and I think he's an incredible person. I've known him since I was 21 years old, you know? He produced my first single. So he's been really good to me. People can say whatever they want.''
The couple actually met when Katharine auditioned for 'American Idol', in which he was a guest judge, back in 2006 but she had no idea who he was at the time.
She said: ''I grew up listening to Céline Dion and I love Andrea Bocelli -- I knew 'The Prayer' because I'd sung it for something and my mom made me learn the Italian.
''I remember calling [my mom] and I was like: 'You're never going to believe, the guest mentors this week is Andrea Bocelli and there's this producer that I don't know but he's supposed to be really amazing. His name is David Foster.'
''My mom was like, 'Katharine, you know who David Foster is. You've listened to every artist that he's ever created.' And I was like, 'Ohh... okay.' ''
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