He's completed most of Nirvana's lawyer training program already. It doesn't take him very long to finish, as Nirvana counts time. A few decades, at most.
A few decades is a very long time when your best friend is being tortured in Hell.
He drills case precedent. (In his free time, he looks up obscure legal citations in volumes no one's touched in centuries.) He rehearses standard and less-standard arguments. (He rehearses what he would even say to him, if he gets the chance.) He takes part in mock trials with the rest of his cohort. (They bring in volunteers from the more experienced students to portray the other afterlives; he's silently grateful, and silently very confident that he'll never be able to pass the favor on, not even if it's just pretending to be Heaven.)
The training program brings in all kinds of guest speakers -- azatas from Elysium, angels from Heaven, even once a modron. Former residents of the Isle of the Penitent. Former advocates who burned out. He pays close attention to every one of them, and silently wishes that they could move a little faster.
As he's approaching the end of the program, they start to let him take on trials of his own. They would do it sooner -- there's no substitute for arguing real trials -- but Pharasma's courts allow but a single advocate from each afterlife, and it's hard to countenance risking someone's eternity to train a lawyer a little faster.
In his earliest trials, he's assigned to unambiguous cases, so clear that even Elysium and Heaven rarely bother to send a representative. He drills his arguments beforehand, just in case someone shows up to raise a counterargument. He and his classmates go over all of their case transcripts afterwards, discussing they arguments he made and finding ways to improve. He gives one of his classmates a citation suggestion, and she gives him an odd look and asks where he even heard of In re Stone.
It's a good suggestion, if niche. She rewrites her argument.
By the time he's nearing graduation, he's assigned slightly more complicated cases. Still nothing where anyone is at risk of eternal damnation, of course, but cases where the outcome isn't predetermined. A cleric of Jaidi who could be lawful, if you look at it the right way, whose only preference between Heaven and Nirvana is the desire for her still-living children to end up wherever she does. An unexceptional employee of an orphanage, the sort of person who would usually be sorted True Neutral but shunted into Nirvana, save for the fact that he's good enough at childcare that Pharasma might actually want them for the Boneyard. A teenager who's clearly tilting Chaotic but desperately wants to stay out of Elysium.
They don't all get sorted to Nirvana; that's to be expected. If he never took on real cases, he'd never learn.
Nirvana's lawyer education program has a tradition that the very last case you take before you graduate is a case where someone is probably doomed to the Evil afterlives. It's important, in this line of work, to know whether that's the sort of outcome you'll be able to endure.