Labrador foraging for food

Question
Hello, I walk a labrador retriever who is under fed by its owner.  The dog is extremely active.  The dog when walked, if not chasing something, it is constantly searching for food.  Tree bark, bird food, dog poop are common eating material. It does not play with the other dogs it simply searches for food. The dog is fed one cup of kibble in the morning and one cup of food in the evening.  The owner is very proud of his "thin" lab regardles of the dog clearly fixating on food.  Is there an appropriate amount of dog food for the labrador?  The owner does not believe the amounts listed on dog food labels because they want your dog to eat more so that you buy more. The poor dog will also harrass strangers walking their dog if they have treats in their pocket. The dog is leashed if we perceive any problems coming. I also walk with two other labs and they do not behave like this.  Any suggestions are welcome!  Thank you.

Answer
Can you see the dog's ribs?  If not, chances are it is at a healthy, lean weight.  A dog definitely should be narrower at the waist than the hips and chest.
You should be able to easily feel the ribs, but not see them.  Each dog is
different.  Standard recommendations are a good place to start, but each dog
must have its food adjusted to its individual needs. Here is a link to a good illustrated guide,
http://www.longliveyourdog.com/twoplus/RateYourDog.aspx

Most Labs need more than 2 cups of food a day.  If the dog is too thin by the link above, you should call the owner's attention to it.  Otherwise, be prepared to give the dog a correction when it tries to forage anything.  I finally understood Labs when I took Pepper to the zoo.  At the seal exhibit, I realized they think they need the same build for swimming in cold water.  I hurriedly moved on when Pepper barked back at the seals.