“Why I have saddled myself with two men as like as not to attempt to walk off an attempt on their lives, I will never know,”

is absolutely not true. Petrana could know the answer to that very easily, with a good hard look at herself in the mirror, and for presumably obvious she chooses not to. Why tell yourself about yourself when you can do absolutely anything else.

Only when she's assured herself that he isn't likely to fall if she lets go, she rifles about in the hidden pockets of her skirts until she can find the vials of poison antidotes that they had been exhorted to carry, mentally running through a checklist of the commonplace poisons they had anticipated encountering. Cross-referencing that checklist with the symptoms he reports, and that which she can observe, and finally:

“All of this,” she instructs, opening the vial and pressing it into his hand so he can still feel like a big man who can drink his own antidote without assistance. “Now, please.”