Final answer:
Agatha would inform her friend of their foul odor if she noticed their breath smelled like garlic and trout because she believes in being honest with her friends.
Step-by-step explanation:
In the given text, a character named Adoniram notices a foul odor and asks his wife about it. The reason Agatha answers that she would inform her friend of their foul odor if she noticed their breath smelled like garlic and trout is not explicitly mentioned in the text.
However, it can be inferred that Agatha believes in being honest with her friends and would let them know if their breath smelled bad, using the example of garlic and trout as a specific instance.