Honor and reputation are incredibly important, even vital, in a medieval society. However, "Duels were fought not to kill the opponent but to gain 'satisfaction', that is, to restore one's honor by demonstrating a willingness to risk one's life for it." ( Duel - Wikipedia )
Have you considered whether Certamen might be more appropriate? From the Core rules (p. 89): Certamen “serves as a nonlethal way for one magus to establish dominance and precedence over another, and has formal restrictions to keep it from disrupting the unity of the Order.” A public Certamen is certainly a good way to show dominance, and you get to cast a spell at the loser.
A WW lasts for a full month and has the potential to escalate by drawing others into it, so it's a lot more risky for the NPC. It's not completely out of the question though: "A declaration of Wizard’s War does not always mean the aggressor actually intends to kill his opponent. [...] Sometimes, the intent of a Wizard’s War is simply to force one’s opponent to negotiate. Wizard’s War can also help persuade the opponent’s covenant to get involved in resolving the dispute." [HoH:S, p. 24]
If the PCs think there's a risk of a WW, they might try to avoid it. "Magi who feel threatened by adversaries [...] sometimes seek the protection of a more powerful magus. The strong magus makes a public pledge to avenge the weak magus if the latter is killed in a Wizard’s War." [HoH:S, p. 24] If the NPC is percieved as a bully, that might be sufficient to get the protection of a more powerful NPC mage. It would certainly make sense for the other PCs to publicly declare their support, in order to prevent a WW.
If you are set on having a WW, then to me this would mean the PCs have one month to get an AC and as many Sympathetic Connections as possible and try to strike first. The other guy probably already has an AC to the PC if he declares WW. He'll probably declare WW on the whole covenant, as John Doe explained.
One strategy is to not go for a kill right away, but to (multi-)cast fatiguing spells (PeCo base 10, but PeIg only base 4!) or a sleep spell with high Penetration right before the other mage's Parma expires, so he falls unconscious and can't renew it. For that you only need an Intangible Tunnel of level 5 and a Touch-range version of Winter's Icy Touch (PeIg 5) or Call to Slumber (ReMe 5). When his Parma is gone, you can follow up with more powerful spells with less penetration. Again, you don't necessarily have to kill him, depending on your goals. Moon-duration PeMe spells are an idea. His friends might dispel them, but the message is "I could have killed you, but didn't." There is a whole chapter "The Intangible Assassin" in the book "Hermetic Projects" that might be helpful.
So to answer your question: The best way to get the other PCs involved is the preparation. They have a whole month to come up with a plan, negotiate support, maybe try to steal the NPC's AC to the PC mage, get an AC of their own, set up wards etc.