I would assume yes, but cannot say for sure.

The answer might be, it works on any NPC that is on a faction that can improve.
Not sure if the guild perk specifically gets you the discounted value or if it gets you the maximum faction discount; the wording will likely be important.
That also assumes your NPC is not on a faction in which players can raise their standing... if they are, than either wording would likely get you the discount.