The description of the Coerce activity says that you can target any creature:
With threats either veiled or overt, you attempt to bully a creature into doing what you want. You must spend at least 1 minute of conversation with a creature you can see and that can either see or sense you ... (CRB, pg. 247)
Player characters are creatures, so it definitely seems that a PC can be targeted by Coerce.
... but not really
The Coerce rules also reference the "Changing Attitudes" sidebar on page 246-247. This sidebar makes it clear that while players may use social skill checks to measure how influential their character is, ultimately a PC is governed by their player's choice - not the die roll.
... You can roleplay interactions with player characters, and even use Diplomacy results if the player wants a mechanical sense of how convincing or charming a character is, but players make the ultimate decisions about how their characters respond. (CRB, pg. 247)
So why have Haughty? First, NPCs with this feat limit how often a PC can attempt to Coerce them. There isn't any apparent mechanical benefit to a PC from this clause, because the player is responsible for determining their response anyway.
Nonetheless, there could be some benefits in skill challenges where a Coerce action is used in a non-traditional way. Published Paizo adventures often contain mini-games where skills and activities are used in non-traditional ways. For example, in The Flooded King's Court PCs can attempt Perform actions to convince a nobleman to allow them access to a dungeon. They are performing (so the Perform activity is narrative relevant), but the results are different than the Perform action's ordinary results. Although I don't know of any examples using Coerce, there is at least precedent for these kinds of uses.