I don't believe Jaheira is intended to join the Ketheric Thorm fight unless she's in your party at the time; when I played it after clearing the ground floor she says she'll stay to secure the area, leaving my party to go up alone. This is with the Nightsong on our side though, so it could play out differently if you don't have her.
That said, you don't want Jaheira fighting alongside you as she seems really determined to get herself killed; honestly you're lucky she survived to the point you reached, I had to put her in Otiluke's Resilient Sphere to keep her from being curb stomped. =P
