Question

how will posterity remember the poet's friend

Answer 1

by the written documents of his friend

Answer 2

In the context of the poem "Not Marble Nor the Gilded Monuments", the poetic verses written by Shakespeare will survive the ravages of time and the fast moving fire of War; not even Mars, the God of War, will be able to destroy the "living memory" of the poet's friend. On the other hand, gold-plated statues and colossal monuments will be in ruins with the passage of time.
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