Every item in the game has "useability" flags that determine which characters can use it. For example, Carsomyr is only useable by Paladins. There are flags for race, alignment, classes, kits, and stats. Corthala Family Armor can actually be worn by lots of characters besides Valygar (human Ranger, multi-class Ranger, Fighter, or multi-class Fighter of any alignment with Dexterity 18 or higher). There is no flag that says, "Only Valygar can use this item."
There also isn't any flag for the Harper Pin that says, "Only Jaheira can use this item," so the designers did the next best thing: they put flags on the item that allow only a half-elf True Neutral Fighter/Druid to use it. Since there aren't many characters besides Jaheira who fit that description, then for all practical purposes, just about nobody except Jaheira can use the Harper Pin. (Actually, due to an oversight, half-elf True Neutral Bards can also use it. Or maybe the designers think that any character who fits that description must be a Harper.)
If you change Jaheira's class or alignment, she will no longer be able to use the Harper Pin. To me, that's hardly a good reason not to edit her. But I know how to edit items as well as characters, so I can have it both ways.
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I have just edited the Harper Pin for you so that ANY half-elf Fighter/Druid, Druid, or Cleric/Ranger of True Neutral or Neutral Good alignment with 15 or higher Charisma and 14 or higher Wisdom can use it. That should allow Jaheira to use it even if you edit her but prevent most other characters from using it. You could abuse it if you wanted to, but you'd be more likely to abuse Use Any Item if you gave that ability to Jaheira, and I really don't care if you abuse it.
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See the attached file. Just put it in your OVERRIDE directory and it should fix your problem the next time you open the game.
In my last game, I edited Yoshimo to make him a Kensai dualled to a Thief. As a result, he wasn't able to use his own katana. Instead of giving him Use Any Item, I edited his katana so that a Thief or a Kensai/Thief with Yoshimo's stats could use it. I'll upload it if anyone is interested in using it.