。
She mourned the death of her only son.
Literal
She [topic-は] only-son [of-の] death [object-を] grieving-and grieved.
一人息子 ('only son') and the parallel 一人娘 ('only daughter') carry particular cultural weight in Japan, where family lines, name continuity, and inheritance traditionally pivot on a single heir. The loss of an only child is a recurring theme in Japanese literature as the deepest of bereavements. The compound predicate いたみ悲しむ stacks two near-synonyms — 痛む ('to ache') and 悲しむ ('to grieve') — using the connective stem-form 痛み as 'and.' Stem-form linking is more literary/written than the te-form equivalent 痛んで悲しんだ.