Deym you

Technically the doctor diagnosed lariette correctly, it's just doha's healing that saved lariettes life
So I wouldn't call the poor dude a quack

Didn't he say he misdiagnosed her in the prologue or did i read that wrong?
I mean, Doha's job was replenished her exhausted mana from purifying Asrahan... Not healing her.... I bet she assumed Doha was healing her since he was technically a priest and that's what divine power is all about...

(If im not wrong) the doctor says he misdiagnosed lariette AFTER she had already had her results that she would die, in fact we see how lariette changed her hair color probably cuz she had already ran away from Asrahan.
Later on we see Doha that realized (while healing lariette) that there was unstable amount of mana in lariette that would make her body explode so he decides to stabilize her mana bit by bit to have her stay alive for his entertainment (at first , clearly not now)

Now I dunno if ma mind is playin tricks on me but I'm pretty sure that's it

Misdiagnosed.... Means he diagnosed her WRONGLY.... I remember him saying it was not curable....even when her brother was diagnosed, they also said the same thing... Not curable...
From what i can infer, her mana destabilizing wasn't due to the illness, but because Lariette has been using her mana without proper training and education cause her parents deprived her of it.... That's why Doha is helping her train to avoid straining herself further...