It's a matter of how much you feel like hand-waving the process.
I'd say the most straightforward approach is to partake in an activity that Pharasma approves of, namely protecting the sanctity of the "souls should be brought for swift judgement" concept. The Ahmuuth Psychopomp is one that specifically teams up with mortals to destroy undead. Even better, the PC may help by preventing the creation of more undead. "Alas, I am restricted in my actions to merely dispatching of the undead, but the necromancer who raises them is outside my jurisdiction". PC kills the necromancer or breaks his staff, and the Psychopomp is grateful for that intervention that he himself was not allowed to take. That's the more intricate option. The hand-wave one is just to have the players encounter some undead in the campaign, and when they go to battle them, an Ahmuuth is also there in his quest to slay them. They find themselves allies for the fight, and the psychopomp expresses his appreciation with "you have proven yourself an ally and friend to Pharasma. Thanks, bye." and the requirement is met. But that general concept is what I'd go with. If you want to avoid throwing in more combat, maybe there was a fight with a demon years back, and he unhallowed a graveyard in the process. Demon is long gone, as are the heros who vanquished him, but now over the years of subtle unholy energy permeating the graves, some of the bodies have begun to rise as completely mindless zombies, and the PCs need to hunt down a priest who can provide a scroll of Permanent Consecrate that needs to be placed on the central tombstone. Just some various ideas.