In the final stanza of "Ode on a Grecian Urn," John Keats refers to the urn as "a friend to man." Three ways in which the urn is a friend are contained in the stanza.
Keats refers to the urn's "Attic shape," "fair attitude," "silent form," and "Cold Pastoral." He refers in general to the men, women, and flora depicted in the urn's drawings and states that the urn and its drawings "tease us out of thought as doth eternity." He means that the urn provides an ongoing source of contemplation. Just as men can never fathom the concept of eternity, so too can they never exhaust the rich source of imaginative speculation that the urn provides. In this way, the urn is a "friend to man" because it continues to enthrall his mental faculties.
In addition, the urn is a friend to man because it will not abandon him. Since it has already endured for centuries and will continue to exist, without dying or aging in the way humans do, it offers steadfast companionship not just to individuals, but to the whole human race. It will outlast the current generation and still be around to please people to come.
Finally, it is a friend because it speaks words of advice to humanity. Whether one agrees or disagrees with the sentiment the urn expresses, it does "speak" a message to humans, and in that way it acts as a friend.
The "Cold Pastoral" is a friend to man because it stimulates man's imagination, remains a steadfast companion, and offers an insightful message.