If the flowers Drew received today had been sent by someone who knows Drew well, that person would have known that Drew prefers violets to roses.
Conclusion
The florist must have made a mistake: either Drew was supposed to get violets, or a card, or the flowers were for someone else.
Evidence
If the sender knew Drew well, they would know that Drew likes violets more than roses.
But the flowers Drew received were roses.
If the sender didn't know Drew well, they would have sent a signed card with the flowers.
But there was no card.
Evaluate
Given these conditional rules and facts, how could we still argue that the flowers were not a mistake, they were violets with no card intended for Drew?
There were two conditionals, and two facts that seemed to trigger their contrapositives.
Sent by someone who doesn't know D → card
The fact that there was no card means that the person who sent it was someone who does know Drew well.
But doesn't that contradict the other conditional?
Sent by someone → would know that D who knows D well prefers violets
While it's counterintuitive, there is no contradiction. The conditional is only saying that the sender would know that Drew prefers roses, not that the sender would have definitely sent roses.
Maybe the sender knows that Drew hates roses and sent him roses sarcastically, to troll him.
Goal
The loose end in this argument is that someone who knows Drew well may have decided to send Drew roses (maybe they were on sale) even though that person knew that Drew prefers violets.