"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.
How do you construe/analyze "sepulchrō," though?
Could it be a dative, with the meaning, "His client is for the grave ", in the sense of "at death's door" (= about to die) ? Of course, I have no idea if such a thing would even make sense, in Latin.
But to get the meaning needed here, "in the grave," we need the preposition in + ablative case: "in sepulchrō."