I think of the Paladin's extreme religious commitment as a focal point for the character. Maybe look at the Domain powers associated with his/her diety, or on a similar track, the Warpriest blessings?
I really like the look of some of the Diefic Obedience boons, as well (such as the Sentinel ones from Iomedae or Shelyn, for example). Could you perhaps tie those into this extra special ability? Maybe start with Diefic Obedience as a bonus feat, then get earlier-than-usual access to the Sentinel boons (perhaps on the same sort of progression as if you took the PRC.... without having to take it)?
What diety is the Paladin worshiping, Diachronos?
That and any other details about her background might also help us with some of the 'flavour' around the special ability.
Well, your character certainly had some contacts and skills specific to her old life as a dealer, has she just forgotten them? Maybe some bonus class skills, skill points or black market contacts of some kind? The kind of thing she would be very embarrassed about, and loath to employee, but could be very helpful.
Going with the Deific Obedience idea, you could work something like the following, which gives the Obedience for free, and then gives early access to the Sentinel boons as if she took the PRC after 5th (but without having to do so -- allowing her to progress as a Paladin for other features):
1st Level - Deific Obedience as bonus feat.
7th Level - Boon #1: Knight of Valor (Sp): bless weapon 3/day, bull’s strength 2/day, or magic vestment 1/day
11th Level - Boon #2: Valorous Smite (Su): If you have the smite evil class feature, you gain an extra use of that ability per day. ... If you successfully deal damage with your smite, your target must succeed at a Will saving throw (with a DC equal to 10 + your Charisma modifier + 1/2 your Hit Dice) or be stunned for 1 round plus 1 round for every 4 Hit Dice you possess (maximum 6 rounds). Once a target saves against this stunning effect, it is immune to the stunning effect from your holy smite for 24 hours.
14th - Boon #3: Banishing Strike (Sp): Once per day, you can channel the effects of banishment through your weapon, though you don’t need to cast (or even know) the spell. You must declare your use of this ability before you roll your attack. On a hit, the target is affected by a banishment effect. If you openly wear a holy symbol of Iomedae, you gain a +1 bonus on your caster level check to overcome the target’s spell resistance (if any) and the saving throw DC increases by 2.
If that seems underpowered, or too skewed towards late-game benefits, maybe consider swapping some less desirable Paladin abilities at lower levels with somethin from the Sentinel PRC (maybe swap Divine Bond at 5th for Symbolic Weapon, and have the Symbolic Weapon progress from 5th onwards).
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Alternatively, because of the background, as well as the relative absence of healing/condition removal in the group, maybe boost her Mercies to signify the healing power granted by Iomedae to balance out the harm she has previously done as a drug dealer.
For example, rather than granting access to one (1) Mercy at 3rd level and every three (3) levels afterwards, maybe grant two (2) at each stage.
Another idea might be to grant the Disease mercy as a bonus, and have it work automatically (rather than having to make a caster level check). This would play in thematically with the drug-dealing (with drug addiction being much like a disease for game purposes). It might also have value given the urban themes of the AP.
If these abilities seem too tame relative to the other characters' special abilities (the Magus' many bonus feats, for example), perhaps you could look at gaining extra uses of lay on hands, or perhaps free access to the Greater Mercy and/or Ultimate Mercy feats (at appropriate levels).
I would go for something simple and related to their devotion as well. Taking the blessings (like a warpriest) for one of Iomedae´s domains would be a good fit imo. Giving her a cleric domain or an inquisition (again, from Iomedae´s list) can also be interesting. I am also a fan of paladins being able to call on celestial allies when they need to, which one of the archetypes (Empyreal Knight) can do, although technically a few blessings can get something like it as well.
As others noted, you can also have something related to her past, but it should not step on what the rogue is getting. Domain, blessing or an inquisition would be a much better fit imo. Having a favored community (like the urban ranger) is an option, but I think a divine-flavored boost would be a better fit.
Nice! I think that approach will work out well for her.