Hilary Duff is officially divorced. And she celebrated by taking a trip to Maui — with her ex-husband. On February 4, Duff, 28, and former NHL star Mike Comrie, 35, jetted to the Hawaiian island with their 3-year-old son, Luca, for a planned eight-day stay. “The vacation is for Hilary to get away, and Mike helps with Luca,” a pal tells Us. “They’ve established a great family dynamic.” Indeed, the source tells Us there’s no bad blood between the Younger actress and Comrie, who finalized their divorce January 28. “They’ve been able to stay friends,” says the insider. “They want to stay close for their son.” MOVING ON The pair, separated since January 2014, will continue to share custody of Luca, and Duff paid Comrie $2.4 million to keep their…