A good poem about friends growing apart is "Shake Hands" by A. E. Housman. This short poem addresses the pain of parting from someone who is no longer a friend and ends with a promise that the narrator will always be there for the former friend.
The first stanza talks about the friendship going sour and begins, "Shake hands, we shall never be friends, all's over; I only vex you the more I try." But the second (and last) stanza says that if the person the poem is addressed to is ever in trouble, "Be good to the lad that loves you true/And the soul that was born to die for you/And whistle and I'll be there." Housman's romantic poetry cycle "A Shropshire Lad" was published in 1896.