The Revenant producer told PEOPLE at the Venice Family Clinic's Silver Circle Gala in Beverly Hills on Monday that Carey and new fiancé James Packer "are both really generous and kind people."
While Ratner, 46, has friendships with both, he says he didn't directly play matchmaker – "it organically happened, which is such a beautiful thing."
"Mariah's my friend; she's always around," he explains. "James is my partner."
He adds, "They hung out as friends first so that was a beautiful thing and beautiful way to get together with somebody. And I'm happy to be in the middle of it!"
Carey and the Australian billionaire first met in 2014, but didn't begin dating until last summer. Packer proposed with a massive 35-carat ring in January – just over a year after Carey and husband Nick Cannon filed for divorce.
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Ratner can't help but gush about Carey, and calls her "amazing."
"She's been a friend for so, so many years," he tells reporters. "I'm not allowed to say how many because that will age us. But I call her my Aries sister because we're born like a day apart. And she's such a great friend. I directed most of her music videos, at least the best ones."
So will the Hercules director be there when the pop diva walks down the aisle? "Hopefully."
He says Carey will be a "gorgeous bride," adding, "she's done it once or twice so I think she's had some experience."
Carey joined Ratner on stage at the Monday event, where they auctioned off four VIP tickets to her Las Vegas show – but did not auction off a date.
"No one in this room can afford a date with you. That would be too expensive," Ratner joked.
"I would gladly do that. That's fine," Carey gamely answered back.
"You're taken!" Ratner replied. "You're engaged to my partner. We can't do that, he'll kill me."
Reporting by CHRISTINA DUGAN