I don't know why you'd consider this a collaboration. They inserted a quote form a conversation with him at the front of their song. Big whoop.
If you're interested, some details were found here:
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At the Memories… Do Not Open Listening Party at 837 NYC, The Chainsmokers member Alex Pall told the audience about this spoken intro:
"That was actually Bono, who we’re lucky enough to have — I don’t know how we got lucky, but he’s a friend of our’s and he’s an incredible person and we’ve been really lucky in the past year and a half to meet incredible artists, like Ryan Tedder, like Chris Martin, like Bono and they’ve really been helpful to us [by] kind of showing us […] the way it is, the way it happens, the way it goes and offer us advice about their journey, about how we should appreciate our journey. And we asked Bono if we could have a call with him and just talk about anything — anytime we have drinks with him, we get into these really deep conversations, so we spoke with him for like 45 minutes and in that 45 minutes we got this perfect quote of his about how it’s the artist’s duty to out themselves. And it’s important, we felt like this really ran true with the song “Honest,” so we put it in there."
CHAINSMOKERS via Facebook
“Honest” is the oldest song on the album. Outing ourselves as one of our mentors advises. It’s a story about a balance between conscience, loneliness, and temptation told through the eyes of one of us after a night out on the road. Drew is talking to himself about what he wants to say to his girlfriend at the end of night when he knows she’s expecting to hear from him. He’s telling her he doesn’t want to lie to her and say he doesn’t get caught up in the superficial world of fame and fortune and what comes with it. He wants to be honest, which means having to admit not always thinking of her. It’s hard to put out a song like this. The message is dark and revealing of a personal moment, but it’s real for anyone who has ever thought about someone outside their relationship. We love this song because it challenges the thought that love and temptation are mutually exclusive. We all want to be happy and in love, but in reality of most relationships, there are still times of darkness, loneliness, and temptation despite our love for the other person."