I've played Baldur's Gate II at least 15 times ...
My approach to resolving buggy quests in BGII has always been firing up a save game editor and manually setting what variables need to be set... but I'm not sure if there's an updated save game editor for the EE yet, nor do I know the variables involved in that quest...
It's not a variable, nor a quest issue. The issue is that the dialog is broken. B.R.O.K.E.N. No amount of editing of saved games will fix that.
If the variable check is done then it could be pretty easy:
Variable that says conversation is not done yet = proceed to broken conversation
Variable that says conversation is already done is set = ok, continue with next non-broken conversation
Problem if it breaks before it checks that variable. Suppose you could just cheat to kill the NPC and set whatever quest variables you need to proceed.
Editing quest variables to fix broken conversations is a thing I remember having to do a bit too many times in Baldur's Gate II over the years... sadly not in Trademeet so I don't know how to fix that particular quest.
Maybe upload the save game OP?