"His client is in the grave."
Translation:Cliens eius in sepulchro est.
Since the sentence is so out-of-context, it's probably better to stick with the 'rule' that reserves the possessive adj. suus, a, um for "belonging to the 3rd person subject." Note that cliens is the subject here (not some "he" or "she"--since we have no other sentence). The genitive eius (from 3rd pers. pronoun is, ea, id, "he, she, it") makes more sense here, I think.