I think "like Achates, faithful to the tomb" means "like Achates, who was [Aeneas's] faithful friend forever". Just as Achates would join any fight Aeneas was in, so too the attorney would join any legal fight his clients were in. A paraphrase of the sentence might be: "Only the attorney was amused, because no matter what the dispute was about, it would be settled legally, and he would faithfully support his client's interests, just like Achates would fight for Aeneas no matter what."