The lawyer argued for his client's innocence.

Literal

Lawyer [topic-は] client [gen-の] innocence [obj-を] asserted.

依頼人 ('client') is the specific word for a lawyer's client — 依頼 ('request, commission') + 人 ('person') = 'one who commissioned (me).' Distinct from 顧客 ('customer, clientele') in more business-general contexts.