>>719151651
Because of pic rel. Illithid parasites is a field she studies extensively, that's why you can find 2 or 3 in her lab. Her entire focus is getting her hands on this exceptionally powerful specimen and the only way she doesn't switch to plan B - which is killing Lae'zel / Tav / etc. then manually winkling out the parasite from his head - if she's tricked into believing that's not possible anymore. Then there's nothing left but letting you leave in peace.

>and even if it did cure lae'zel or durge, there are still 3 other dudes and a broken machine
Technically, she only learns about the special condition of the party member who initiated the conversation. There might be a small oversight here (I don't know if the "who goes first" scene is there if you do the whole sequence as Lae'zel) but nothing that would substantially change the situation.