The Jonas Brothers singer was thrilled to be asked by his friend to assist her at the Judges' Houses stage of 'The X Factor' and thinks they work well together because they have such a good understanding.
He said: ''Man, there are so many things that have happened over time with Demi and I. I think that we have a really strong bond, a good relationship over time and have seen a lot of life together at this point.
''There was always sort of that bond that goes beyond words, really, and it's great to see that now seven years into our relationship and friendship that we still have that.
''And today we're looking at each other across the room as the contestants are singing, and I think we knew exactly what the other one was thinking.''
Demi is delighted with her finalists and is confident she can win the show with one of her acts thanks to Nick's input.
She said: ''The reason why I picked Nick is because we kind of grew up together. He's a really smart, intelligent guy and, plus, we just know each other so well and we have so much fun together. I know that I have the winner in my category.''
However, fellow judge Britney Spears is equally confident she and her assistant Will.i.am - who previously assisted Cheryl Cole on the UK version of the show and is a coach on 'The Voice UK' - have picked a winner.
She said: ''Oh, yes. The winner is definitely in my category.''
He said: ''What I witnessed today, especially coming in cold, is Simon stands a hell of a chance, a hell of a chance.''
Fourth judge L.A. Reid was assisted by Justin Bieber and his manager Scooter Braun.
As they do every summer, the Martin family relocate to their summer home over the...