I would think though that based on how the last fight played out Z would now realize that prepping all his spells just for dealing with V is a losing strategy. Wizard with genius level intellect and all that. And even if Z is still completely focused on fighting V, dominating one of V's allies is a perfectly valid anti-V strategy (and nicely pays V back for doing the same to him in their last fight.) And if nothing else prepping that spell leaves Z with a ready counter-spell option in case V tries to Dominate one of Z's allies again.
I don't recall what Z's banned schools are, but if he does have access to Enchantment I would be shocked if he hasn't prepped Dominate Person for this fight. And given Belkar's low will save, and the fact that the spell works through a telepathic link and thus will still work despite Belkar being deafened, there's a good chance Belkar is about to switch sides. And with V MIA and Durkon in melee with a powerful opponent, the Order won't have any immediate way to switch him back.