The actors first met while they were working together on the set of Gossip Girl in 2008. During their time together, they went on a couples’ retreat in Jamaica with fellow Gossip Girl co-stars Jessica Szohr and Ed Westwick. And in 2009, Leighton gushed about Sebastian being her “best friend.” However, the pair called it quits in April of 2010.
As the news got out, one source revealed: “They are still friends. It was amicable. They still talk all the time.”
Source : https://www.distractify.com/p/sebastian-stan-girlfriend