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. Yet Drew received roses. On the other hand, if the flowers had been sent by someone who does not know Drew well, then that person would have sent a signed card with the flowers. Yet Drew received no card. Ttherefore, the florist must have made some sort of mistake: either Drew was supposed to receive violets, or a card, or these flowers were intended for someone else.
Someone who does not know Drew well would be unlikely to send Drew flowers most weakens the argument.