Is a pinned character treated as having 0 Dex with respect to the pinning opponent?

Question

Relevant Rules

Grappling consequences:

You lose your Dexterity bonus to AC (if you have one) against opponents you aren’t grappling. (You can still use it against opponents you are grappling.)

Footnote regarding pinned and helpless defenders' AC in Favorable and Unfavorable Conditions, marked against the -4/+0 modifiers for melee/ranged for each:

Treat the defender’s Dexterity as 0 (–5 modifier).

If You're Pinned by an Opponent:

While you’re pinned, you take a –4 penalty to your AC against opponents other than the one pinning you.

Example

Fiona's BAB grants her multiple attacks. In a grapple, she uses one of these to successfully pin Edgar, and the next to attempt to use his rapier against him. She succeeds on the opposed grapple check and makes an attack roll with the weapon, taking a -4 penalty.

Edgar retains his Dex bonus against Fiona, and does not suffer the -4 AC penalty he would against other opponents. Therefore,

1. As Fiona is the one pinning him, his AC is unchanged (10 + Dex + mods), or
2. As he is pinned, his effective Dex is 0, and a -5 Dex modifier is applied to his AC (10 - 5 + mods).

So which is it?

RAW seems ambiguous to me here, but I am inclined to think number 1 applies. The footnote above is attached to a modifier that is applied to non-pinning opponents. The intention of pinning is probably a "I'll hold him, you punch" tactic rather than locking them down one-on-one.

On the other hand, if a pin is representative of your holding a character so still that their Dex is effectively 0, then he's not moving with respect to the pinner either.

Answer 1

All attackers—not just the pinning creature—treat a pinned creature's Dexterity as 0 for determining the pinned creature's Armor Class

The Player's Handbook's glossary describes pinned as "held immobile (but not helpless) in a grapple." That immobility is represented by treating the creature's Dexterity as 0 for determining the creature's Armor Class.¹

Thus, initially, the grappling creature and the grappled creature lose their Dexterity bonuses to Armor Class against attacks made by creatures not involved in their grapple (PH 156).

Then the pinned creature—who's also grappled as that effect has yet to be remedied—is treated as having a Dexterity score of 0 for determining Armor Class (PH 151), but the pinned creature is not paralyzed (as a creature with an actual Dexterity score of 0 usually is) (PH 304, 311), and the pinned creature is not helpless (PH 153).

Further, the pinned creature suffers an additional −4 penalty to Armor Class against attacks made by creatures not involved in his grapple (PH 156).²

Example

Arboc (base attack bonus +6 and usually Dexterity 13 and AC 20) and Baracs (usually Dexterity 16 and AC 17) are already grappling, making their Armor Classes, respectively, 19 and 14 against attacks launched by those not involved in the grapple.

Arboc, on his turn, wins an opposed grapple check at his full base attack bonus against Baracs. Arboc picks to pin Baracs. Baracs, now virtually immobile has his Dexterity for the purposes of determining his Armor Class reduced to 0, therefore an armor class of 9. Further, Baracs suffers a −4 penalty to Armor Class against attacks launched by those not involved in the grapple, making his Armor Class against those foes 5.

If this seems excessive, try imagining being pinned as failing a second saving throw against an effect. Since failing the first could've killed the creature, probably either grappling is the creature's specialty, the grappling creature wants the grappled creature alive, or nobody's casting spells.

Grappling is one of the primary reasons many high-level creatures have access to some kind of freedom of movement effect (e.g. the ring of freedom of movement (DMG 232) (40,000 gp; 0 lbs.)).

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1. The Rules Compendium defines immobilized as "An immobilized creature can’t move out of the space it was in when it became immobilized. It otherwise functions normally unless it’s flying. Immobilized flying creatures that have the ability to hover can maintain their initial altitude. All other flying creatures subjected to this condition descend at a rate of 20 feet per round until they reach the ground, taking no falling damage" (35). The RC doesn't link that condition to the pinned condition, however, stating separately that a pinned creature "can’t move, so its Dexterity is considered to be 0 for the purpose of determining AC (−5 modifier)" (35).
2. I speculate this represents the pinning creature's ability to steer the pinned creature into oncoming attacks. D&D 3.X, so far as I'm aware, omits a grapple benefit that permits actively employing the pinned creature as a shield or as cover.

Answer 2

The answer is number one, because as usual text trumps table. And this is what the text has to say about it.

When an opponent has pinned you, you are held immobile (but not helpless) for 1 round. While you’re pinned, you take a -4 penalty to your AC against opponents other than the one pinning you. At your opponent’s option, you may also be unable to speak. On your turn, you can try to escape the pin by making an opposed grapple check in place of an attack. You can make an Escape Artist check in place of your grapple check if you want, but this requires a standard action. If you win, you escape the pin, but you’re still grappling.

Source (Emphasis mine.)

Furthermore, being pinned is a special case that trumps the general rules of being grappled.