ANSWER: Slate stole the secret code when he visited Crypto Cat at 6pm!

EXPLANATION:
Draw a grid like the one below. Place an X in the box where an impossibility occurs. Place a O in the box where a friend occurs.

Logic Grid
  6:45PM 7:20PM 8:25PM Drink BandAid Towel
Slate O X X X O X
Joules X O X O X X
D. Dog X X O X X O