Lae'zel is still helping you, that she's doing so reluctantly is irrelevant.
If Lae'zel did not know the tadpole was a threat, she would not seek a cure, and therefore not need you at all. She will fight you or ignore you.
Actually Lae'zel does know it's a threat, the Gith have been slaying Mindflayers for how many centuries, pretty sure they would have the most knowledge on tadpoles and ceremorphosis, also if the Gighyanki didn't have the knowledge, then their more heroic monastic idelogucal cousins the Githzerai would know about it.
BTW if you think Mindflayer is bad, wait until a Dragon goes though Ceremorphosis,
you'll get a Brainstealer Dragon.
