Jaheira is by default a druid/sorcerer and thus will benefit from it. .
Why would she be a Druid Sorcerer? She's always been a Fighter Druid.
And... do we know that spell progression scales across muliple classes? I doubt you'd get level 6 Druid spells with one level Druid. I may be wrong, but I just cannot fathom it working that way.
Edit:
How I think they implemented is; you get one spellbook. Gain one level of X - That's your spellbook, modified by your ability score Gain one level of Y - That's still your spellbook but with more spells modified by your ability score and/or your ability score Gain one level of X - That's your spellbook, added spells modified by your ability score and/or your ability score Gain one level of Y - That's still your spellbook but with more spells modified by your ability score and/or your ability score
