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"They are great friends," a source tells PEOPLE after the Wanderlust costars dined Wednesday at Los Angeles's Sunset Tower Hotel, a favorite Aniston spot. "When Justin is in L.A. from New York, he and Jen have spent a lot of time together lately."
The two arrived separately, though the source says they came from Aniston's house, where they also returned after their three-hour meal.
"Jen was in a great mood and seemed happy to take Justin to her favorite restaurant," the source says. "They seemed happy and were flirty, but didn't act like a couple."
According to the source, the duo have spent a lot of time together since March, when Aniston, 42, was spotted visiting Theroux, 39, at the Chateau Marmont in L.A.
A rep for Aniston tells PEOPLE of Wednesday's outing, "It was a group dinner with six people. It was not a romantic dinner." News of their dinner was first reported by X17.com.