He knows
He does? I thought he only thought she was a lackey/dog and not actually part of the family?(also if he did know she was part of family then he would know she knows the flowers are poisonous.)
He knows because he knows Roxana's mother and brother. Also, the servant calling Roxana 'lady' (I'm not sure about the lady, but something like that). Well, I think, he is just a bit naive or maybe there are places or other family that also produce that kind of flower which Roxana didn't know. (Well, I'm just imaging possible scenarios)
Cassis underestimates her so fucking hard this embarrassing. (From my information he doesn't know she's an agriche right?)