"This is a big event for Prince Harry, because it's one of his best friends getting married," Nicholl says. "It's a big deal for her to be going as his plus one. This wedding will be a huge amount of fun. There will be a lot of alcohol going down."
The trip might be a good indication that Harry and Markle are getting closer to their own walk down the aisle.
"I'm hearing from many of his friends Harry could be popping the question to Meghan by the end of the year," Nicholl says.
Harry will be the best man at the Caribbean nuptials between the groom, Tom "Skipp" Inskip, and the bride, literary agent Lara Hughes-Young.
The bachelors are expected to run amuck, as Inskip has a reputation for being a bit of a party animal.
"Whenever there's trouble, Tom Inskip is never very far away from the drama," Nicholl explains. "He was with Harry in that Las Vegas room when Harry was stripping during a game of billiards. He's very into practical jokes, which Harry absolutely loves."
ET has confirmed that the whole crew will be staying at the Round Hill Hotel and Villas. The 110-acre resort includes rooms that cost between $600 to $7,000 per night.
"I'm told [Harry and Markle] are certainly sharing a room together and that they've gone for one of the top-end villas at the resort," Nicholl says. "And they'll have the luxury of a private butler, who will cook them breakfast."