Lovato attended the luncheon in Hollywood in honour of her Seventeen magazine February cover issue on Wednesday, but as Kardashian was unable to attend she opted to send a letter instead which was read aloud to guests, says E! Online.
Kardashian's note said: "Thank you for always being a true friend, for always following your heart and for being an inspiration to me and countless other girls around the world. Your strength and kindness will never go unnoticed."
Lovato had discussed her friendship with Kardashian in her interview for Seventeen, saying that the reality star had provided support during her recovery from emotional and physical issues last year.
"Kim Kardashian, the busiest woman on the planet, took time to call me," she said. "Kim was there for me when not many people were. I could count on my hands how many people were there for me. So the people who were, I didn't forget it."
At the same luncheon a letter from Lovato's close friend Selena Gomez was also read, in which the teenager expressed her respect for Lovato's courage in addressing her problems and becoming a role model to others in the same situation.
Lovato had told Seventeen in the same interview that Gomez would always be in her life, alongside fellow Disney star and close friend Miley Cyrus, thanking them both for their support over the last year.
"There were two people [who were there for me]. No matter how many times we fight, we will always be in each others' lives," she explained.
"[Miley and I] are very strong-headed, so when we fight, it's brutal and we're like, 'We're never going to be friends again!' Then two days later, we're like, 'I love you and I miss you!'
"Selena and I, we never fight. She's not a confrontational person at all. Both of them will never leave my life no matter what happens, because we've been through so much together."
Lovato can be seen at next week's People's Choice Awards (January 11 on CBS), where she is nominated in the 'Favourite Pop Artist' category and scheduled to perform.
> Demi Lovato: 'Kim Kardashian, Selena Gomez supported me in rehab'
> Demi Lovato: 'I'm thankful at how far I've come'
Watch Demi Lovato's video for 'Skyscraper' below: