Interesting. Weird, but interesting.
I apologize for bumping this old post, first of all. This happens to me in my current game, where nothing happened right after Arilistan dies. Since this wasnt answered anywhere else, I thought I'd share how I fix this problem
It took me a while to figure out what went wrong.. The Area file for the Cave (AR2602) does not have any script attached to it for some reason.
To fix this, you'll need a savegame before entering the Forest of Tethir. Load up an Inifinity Editor (either DLTCEP or Near Inifity). Open the area AR2602. Attach the script AR2602.BCS to it. Save AR2602.ARE in override. Open your BG2 cache directory (usually BGII - SOA\cache). Delete everything that has AR260x.bif in it.
Load up the pre-Tethir savegame. This is important because if you load a savegame while in the forest of tethir, it will just load the area from cache or something. Everything should work correctly when Arilistan dies.