Question:
Suppose a party has a Dustman - and said party comes under attack by a horde of ghouls (though, they don't attack a the Dustman until he attacks them first) - the Dustman helps the party both physically and via spells to dispatch the ghouls...
has he broken the undead truce? eg. If the party encounters other hostile undead, will they now attack the Dustman as well?
I would say that, first, the Dustman should not be helping their companions against the undead in any direct manner - for, yes, that is breaking the truce, which should be an important thing to the Dustman ... the truce means something because both he and the undead believe in it ... if he goes breaking it whenever he likes he has proven a lack of belief and respect for the factions philosophies/traditions. If he does it once we call it an infraction and no big deal ... but if he continues to do so then he proves that his belief is not what it should be and loses the benefits of the faction. When his superiors discover as much, he will be officially kicked out - at the least.
You remind the player that the truce is so strong that even free willed undead will not break it; whether or not they might like to in a given situation. The benefits gained by faction beliefs need to be held strongly or they are lost and you are, effectively, no longer a member of said faction. (the lowest level members of factions, namers and the like, who are mostly there to gain the social support/protection a faction offers, should not actually gain those benefits themselves ... until they've proven themselves enough to be fully indroctrinated)
As GM you should be able to determine at which point he has foresaken his beliefs enough through his behavior ... if he did this numerous times throughout the night, that is really pushing it ... if he broke the truce this fight, then goes right ahead and breaks it again during the next, that is certainly reason for expuslion ... if he did it this once, feels wrong about it and doesn't do it again, then it has potential to improve rp a bit as he wrestles with the decision and should be left at that.
If the Dustman wanted to protect his companions from ghouls, he wouldn't have to actually attack them; he could simply use spells such as command undead or halt undead, or he could rebuke them. I can't imagine that these would violate the Truce, since Dustman necromancers would be pretty useless without them. But what if the Dustman uses a non-necromantic non-lethal spell, such as web? Does the Dead Truce consider that an attack?
Probably okay, as long as the group then moves on and doesn't take advantage of the spell to inflict any damage on the undead. There will be judgment calls necessary. The core idea is that the Dustman should be treating undead with the respect most people would accord sentient beings, even something approaching reverence for the purity of their existence.
Occasionally skirting the line is a part of dealing with the complexities of the world. How forgiving the Truce is of those brushes is up to the GM. The more important question is often whether the Dustman is looking for ways to skirt the line. If he is, he's already broken the Truce in spirit, because he doesn't understand that the stricture should apply to him.
A paladin subscribes to the code that torture is wrong. He needs information, wrestles with his conscience, and attempts tougher and tougher measures on a prisoner, agonizing at every step over what it's right to do. Eventually he crosses the line and loses his powers. This is behavior the atonement spell was made for. On the other hand, if he hires a Guvner sage to write out a pre-reasoned explanation of why it's okay to do this, that and the other to prisoners ("Sure, repeated rays of exhaustion are fine, they don't do any real damage!"), heck, I'd probably smack him with power loss right there. He's missed the point -- honorable behavior doesn't include looking for ways to justify torture. You look for ways to avoid having to do it. (And generally speaking, "It usually has negative effectiveness because it's more likely to provide false information" is the best reason not to anyway.) He likes the rules, they're there for good reasons explainable by wise people. They're proper things to aspire to, not impositions to chafe under. Similar with the code of behavior the Dead Truce requires of a Dustman.
A paladin is supposed to stop associating with his companions if they consistently offend his moral code. That, of course, doesn't mean that his companions are actually bound by the same code; it just means the paladin won't tolerate obvious or flagrant moral violations. The paladin isn't going to stand for the rogue mugging someone right in front of him, but if the rogue slips off into the shadows and comes back with the key to the fortress, the paladin might decide he's better off not asking questions about where it came from.
The Dustman should probably use a similar standard for his companions. If they're always eager to whack the heads off of zombies, the Dustman is going to ditch them and find a new adventuring party. So, any good Dustman (or paladin, for that matter) is probably going to set expectations for his companions before they go on an adventure.
"OK, guys, we're probably going to run into some undead in this tomb. If we do, just stand back and let me handle them. I'll use my staff of necromancy to halt them while you run to the next chamber. Please, try not to attack them unless you have no other choice. These undead are so close to the True Death now; if you kill them, they'll probably be reborn as the living and have to start all over. Zanton, you're a Godsman. Would you like it if someone forced you to de-evolve into an animal?"
He has broken the Truce. Aiding those attacking the undead will permit mindless undead to attack the Dustman.
I believe the Truce is not utterly lost once ever broken, and that the effects of the broken truce will only extend to those undead with knowledge of the broken truce. Mindless undead do not communicate among themselves, this will generally speaking only extend the duration of a single encounter, to those mindless undead that actually witnessed the breaking of the pact. Free-willed undead or a necromancer able to communicate with other undead might be able to spread the word more. On the other hand, you might wish to regard the Dead Truce as something as strict as a paladin's code, and require the equivalent of an atonement to be reestablished in force once ever broken.