I assume it's being used literally, rather than metaphorically, here.
Even if ...
Shouldnt he say just Diaboli?
I dont speak latin ... but those words simply dont fit the scene.
You are assuming a modern translation: advocatus = advocate = lawyer. A more accurate translation of "advacatus" would be agent.
Wyll thinks she is Zariel's agent. He might even be right, given the misdirection and half-truths we have been told about the origin characters.
I was pretty sure that it always meant someone who gave you judical aid. Cicero would be called an advocatus, for example.
But looking at wiktionary, it says it has more meaning:
advocātus m (genitive advocātī, feminine advocāta); second declension
1. One called to aid.
2. (law) advocate, attendant (friend who supports in a trial)
3. witness, supporter
4. mediator
So, maybe it could be meaning one? Still a stretch, imho. Well, Advocatus Diaboli is a rather specific term which fits not at all.
So I still believe it is just bad latin.
