The 'Jealous' hitmaker is a long-time friend of the brunette beauty and to mark Demi's 25th birthday birthday on Sunday (20.08.17), Nick paid a heartfelt tribute to the star on his Instagram account.
Alongside some throwback photographs of the duo - including a shot from their 2008 movie 'Camp Rock' - Nick wrote: ''Happy birthday @ddlovato we've seen a lot of life together already.. much more to come as well. I admire you so much and am so grateful to have you in my life. Wish I could be with you today to celebrate! I know you won't ever let me live it down that you're a month older than me. (sic)''
Demi dated Nick's older brother Joe in 2010, and the dark-haired hunk previously admitted to being an ''emotional bridge'' between his sibling and 'Sorry Not Sorry' hitmaker'.
Nick also believes Demi's relationship with Joe helped to simplify their friendship, because it ruled out the possibility of them ever becoming romantic.
Reflecting on the early years of their friendship, he shared: ''Well, for a couple of years there was the Joe and Demi romantic thing, so I'd be this emotional bridge for them.
''But she and I were productive - we'd write songs about it. We became even closer because it was never going to be [romantic].''
Demi also has vivid memories of the role Nick played in her relationship with Joe.
Speaking in 2016, she recalled: ''There's a song on my first album called 'Gonna Get Caught' about how I thought Joe was a player.
''I remember we were on the bus writing the bridge, and Joe is like, 'I think it should have a happy ending.' I'm like, 'No, I don't think it should.' And poor Nick is sitting there like, 'What are we actually deciding here?'''
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