The odds are irrelevant. The quest is to prove the statement false. So finding a vowel on the back of the 7 card 'cuts the mustard'. Even flipping the 7 to find 'DUMMY' on the other side would render the statement bogus, since there would indeed be a vowel there.

What is "used dog food" anyway? Would it be a sack of canine chow that has been opened, or some of its eaten contents heaved back up, or perhaps 'fully digested'?