Personally i would have used the diet coke opportunity! and i'd have said exactly what you didn't want to say..
A. because it's true
B. because it would make him think twice about saying something like that, and actually realise these drinks exist for a GOOD REASON.
C. i'd have just found it amusing and it's just my sarcastic sense of humour as well
Of course... You could level your pen and a tester kit laying around and when asked... whats this? explain...
or maybe leave your "I am diabetic" wallet card on the table?