I wouldn't say they were best friends but countless times they have both said they enjoy working with each other, Leighton has even said that Ed is her favourite co star as he is so professional and that even she knows that they have amazing chemistry.
Not sure if they're *best-friends*, per say, but they both already said, multiple times, that they're really good friends and that they really enjoy working with one another :)
I'm glad that Leighton realizes they have ah-mazing chemistry. . . I don't think that they're best, best friends. But I think that they get along very well.
