I am aware that Franklin published Whitefield's sermons in his Gazette and Whitefield was happy to allow him to do so. How did their relationship go beyond business interactions?
They were friends with a high level of mutual respect. Obviously there was a big religious difference, but this did not hinder their mutual admiration. There is surviving correspondence, and at least one anecdote in Franklin's Autobiography. Harry Stout discusses their relationship in his biography of Whitfield and in this journal article.