The stunning model showed her former lover what he's missing out on, as she went braless in a plunging white mini dress, as seen in photos obtained by OK!.
Booker on the other hand, looked a bit unkept with an unshaved face, sporting a red baseball cap, white tank top and matching pants.
Rubin shared photos from the star-studded evening on Instagram, highlighting other notable guests in attendance — including Tom Brady, Kim Kardashian, Lori Harvey, Hailey and Justin Bieber, Travis Scott, Ben Affleck, Jennifer Lopez, Leonardo DiCaprio, Beyonce, Jay-Z, Usher and more.
After the Fanatics CEO uploaded pictures from the A-list event, fans quickly noticed the exes' reunion and wondered why Jenner's rumored new beau, Bad Bunny, was seemingly not in attendance.
"Book and Kendall at same party? Where’s sad bunny," one Instagram user quipped in the comments section of Rubin's post, while others pointed out Booker's lack of grooming.
"Book let himself go😭😭😭," someone criticized, as another person wrote, "wtf going on with book 💀."
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Supporters of Booker defended the NBA star, stating that he always grows out his facial hair during the off season and it shouldn't be a cause for concern.
The exciting night in the Hamptons may have triggered flashbacks for Jenner and Booker, as they attended the same all-white party celebrating July 4th last year, though back then, they arrived as a couple.
At the time, the former flames were in the midst of an on-again, off-again relationship after news broke in June 2022 that the duo had split but were later confirmed to have gotten back together, as OK! previously reported.
"Kendall and Devin arrived to Michael's party together and were hanging out by each other's sides a majority of the night," a source spilled last year of Rubin's 2022 July 4th soirée. "They were with a group of friends but they weren't trying to hide that they were there together."
"Kendall was holding on to Devin's arm the majority of the night and they looked really cute," the insider detailed. "They both were hanging out at the bar taking shots of 818 Tequila and Devin was grabbing rosé for them to also drink. Kendall looked really happy and she was smiling and laughing at him throughout the night."
Booker and Jenner officially split in October 2022. The 818 Tequila founder has since moved on to Bad Bunny, whom she was first spotted with in February. The rumored couple even stepped out to the Met Gala together on Monday, May 1, and are reportedly still going strong.