Roger and Rafa definitely have a friendship - I've always found Roger a bit smug, personally, but the interactions they have in interviews and the videos they do are genuine and funny, and you can't fake that. Since Rafa was unable to make the Laver Cup this year, I think it's probably right that Roger be there in Malaga, as a fan- because he is a fan, and a friend.
Serbia haven't made the finals of the Davis Cup, so no reason for Djok to be there as a player - but I suppose he will be expected to turn up. But the story will be Rafa.