They split in May and reconnected temporarily in July, but Paris Hilton and Stavros Niarchos certainly seemed back on at the South Beach, Fla., nightclub Ink on Friday night. Hilton, 25, and Niarchos met up around 1 a.m. with Hilton's pal Brandon Davis and sister Nicky Hilton (who earlier hit Andy Roddick's charity celebrity poker tournament at Hard Rock Live), but only had eyes for each other when the DJ played Billy Idol's "Mony Mony" and Hilton treated the Greek shipping heir to a lap dance. The group later moved on to the club Mokai in Miami Beach, where Niarchos joined Hilton on the dance floor, at one point even lifting her up and spinning her around on his shoulder. Their night hit a snag at around 3:15, when the pair stepped outside and briefly argued about their plans (Niarchos wanted to leave but Hilton didn't plus the barefoot heiress had left her shoes inside the club). But Niarchos apparently won out. So what's the story with Niarchos and Hilton, who was recently photographed with a diamond on her ring finger? Her rep, Elliot Mintz, tells PEOPLE, "Paris is not engaged. She does wear rings on different hands and fingers and depending on what's comfortable, but she is not engaged." Still, a source close to Hilton tells PEOPLE, "Not only are she and Stavros) together, but they are in love."