“You were always a good friend to me,” said Scrooge.
Ebenezer Scrooge speaks these words to the ghost of his former business partner, Jacob Marley, on Christmas Eve, on the seventh anniversary of Marley's death.
The narrator of Charles Dickens's A Christmas Carol tells the reader in stave 1 that Scrooge was Marley's "sole friend." It's likely that Marley was Scrooge's sole friend as well. The narrator mentions Scrooge's "business friends in the city," but it's possible that these "business friends" were little more than associates and acquaintances who counted Scrooge among their "friends" because of mutually profitable business arrangements between them, rather than as a reflection of any true emotional bond of friendship.
Scrooge's friendship with Marley might have been a friendship in name only, as a mutually profitable partner-to-partner relationship. When Scrooge demands to know who Marley is, Marley doesn't respond by saying that he was his friend. Instead, Marley says, "I was your partner, Jacob Marley," thereby asserting the relative importance of their partnership and their friendship.
However, the events of that Christmas Eve when Marley's ghost appears to Scrooge seem to demonstrate that Scrooge and Marley were more than just partners, and they might well have been friends, too, even if in their own ways.
Marley explains to Scrooge that, because of the life he led, he's been doomed to wander throughout the world "and witness what it cannot share, but might have shared on earth, and turned to happiness." Marley says that he's come to warn Scrooge that Scrooge might yet have "a chance and hope of escaping my fate."
Marley says one more, very important thing which is only occasionally mentioned and rarely emphasized in summaries and critical analyses of A Christmas Carol:
A chance and hope of my procuring, Ebenezer.
Somehow, Marley arranges to actually appear to Scrooge in person this Christmas Eve. When Marley previously visited Scrooge he had been invisible to him.
How it is that I appear before you in a shape that you can see, I may not tell. I have sat invisible beside you many and many a day.
Marley knows Scrooge well enough to realize that his appearance alone, even as a ghastly semitransparent ghost with "death-cold eyes" who can walk through heavy doors, won't be enough to cause Scrooge to change his "squeezing, wrenching, grasping, scraping, clutching, covetous" ways, so Marley somehow arranges for the Ghosts of Christmas Past, Present, and Yet to Come visit Scrooge as well.
Marley is deeply offended when Scrooge doesn't take him seriously and says that he'd rather not be visited by the ghosts or that he'd prefer to "take ’em all at once, and have it over." Marley is quick to impress on Scrooge the importance of his own visit and that without the ghosts' visits, "you cannot hope to shun the path I tread."
Marley must have been quite a good friend to Scrooge when he was alive—or learned to be a good friend to Scrooge after his death—to go to the trouble of "procuring" his own appearance as well as the appearance of the three spirits to help Scrooge learn to be a better person and to spare him the suffering and regret that Marley is condemned to endure as the result of his own "life’s opportunity misused."