The cemeteries of ancient Rome were pretty extravagant, and the tombs of the wealthy were particularly elaborate. Resembling the houses of the ancient Romans, the tombs contained multiple chambers, including one in which the family of the deceased would dine as part of a memorial celebration. (https://en.wikipedia.org/wiki/Roman_funerary_practices#Tombs)
Rather than envisioning a hole in the ground containing a coffin and accompanied by a tombstone, I imagine that the sentence is probably referring to one of these ancient structures, and the aforementioned client had simply gone inside to assess the damaged wall that he had hired the mason to repair.
As he examines the growing crack in the tomb's wall, the man reminisces of the times he and his older brother had slipped away from their parents to come play in this very room when they were younger. Of course that was long ago, before his brother had become a permanent resident of the chamber adjoined to this one.
The sound of rapid footsteps behind him draws him out of his reverie, and he turns just in time to hinder the fatal thrust from his would-be assassin, but not in time to avoid it altogether. As the blade sinks into his abdomen, he pushes his masked assailant away. The figure falls to the ground, dislodging the mask and revealing a familiar face.
The client is both bewildered and angry upon learning the identity of his attacker, and as they are enjoined in a physical struggle, they exchange harsh words with each other. Despite his luck at avoiding the full force of the initial stab, the client's wound is ultimately still his undoing, as the loss of blood saps him of his strength. His struggles are in vain, and he is powerless to prevent the blade from being withdrawn from his abdomen and thrust into his heart.
The figure stands and puts the mask back in place. The goal is accomplished, and it is now time to make a quick exit. A sudden fluttering sound startles the figure, who turns in time to see the parrot fly out of the chamber. The figure is seized by panic as snippets of the recent argument are echoed throughout the tomb. Quickly the figure rushes to capture this surprise witness, but it's too late. The parrot flies out of the tomb and into the sky. The figure curses silently. If the parrot's words are heard by the wrong ears, the whole plan could be undone. No time to dwell on that now though, as it sounds like someone else is approaching. The figure retreats in the direction opposite that of the oncoming footsteps, and disappears into the shadows.
After waiting outside of the tomb for ten minutes, the mason decides to go inside to see if his client had perhaps already entered. As he steps inside, he is surprised by the sound of fluttering wings as a parrot enters behind him and alights on his shoulder. The parrot seems a little unsteady, and the scent of alcohol assaults the man's nose. Suddenly the parrot starts to argue with itself. It seems this parrot is from some pretty dysfunctional household. Would a traumatized bird, like a human, perhaps resort to a few drinks to cope? Would it even be able to draw the connection between its drink and the drunkenness that follows?
His thoughts are interrupted upon hearing the parrot say his client's name in one of its outbursts. Coincidence? Then again, he doesn't know too much about his client's home life, and this was his client's family's tomb, so maybe this parrot was his client's pet. It could make things awkward if the parrot continued its hollering, so he shoos it from his shoulder before venturing into the next chamber in search of his client.