Your friend claims that every pound of food you eat results in a pound gained in weight use the law of conservation to explain why this might be a valid statement

The law of conservation of mass states that the mass of the reactants and products are the same. This means that the mass of what you start and end with are the same.

If eating, this means that however much food I start with outside my body, the same amount will be added to my weight because it will be inside my body. The food can't disappear, so it will be added to my weight.

Explanation:

The law of conservation of mass states that the mass of the reactants is equal to the mass of the reactants. This can be seen in a chemical equation, which looks something like this:

A + BC => AB + C                        (This example was single displacement)

On the left are the reactants. On the right are the products.

Even though the chemical composition of the two substances (separated by plus sign +) changed, you still have the same letters on each side.

For example, this would be impossible:

A + B => A                     The reactants are products are different.

So what your friend is saying is, if I eat "A" pounds of food, I will gain "A" pounds of weight, or:

A => A

The mass of reactants equals the mass of products.

Although, whether or not your friend is actually right or wrong is a different story.