Is sweet corn a grain or a vegetable?

You both are correct....

Corn (Zea mays) is sometimes called a vegetable grain. Corn is a monocotyledon with only one seed leaf like grasses. The easily identified "grains" (or cereal plants/grasses) such as wheat, oats, and barley are also monocots. A grain is defined as the harvested dry seeds or fruit of a cereal grass, or the term can refer to the cereal grasses collectively.

Field corn that is harvested when the seeds are dry would thus be considered a grain. Sweet corn when harvested before maturity is usually considered a vegetable. It is grown to be eaten fresh as a tender vegetable rather than as a dried grain suitable for grinding into flour or meal. A vegetable is defined as a plant cultivated for an edible part or parts such as roots, stems, leaves, flowers, or seeds/fruit.

If you want to be very precise, all cereal grains could be called vegetables, but by convention we separate the cereal grains from the rest of the "vegetables" such as peas, lettuce, potatoes, cabbage, etc.

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It depends on how you define vegetable. Botanically speaking, corn is a particular type of dry fruit from grasses called a caryopsis. Whereas a vegetable is basically any edible plant or part of a plant that is not a seed or a sweet fruit. So techinically sweet corn is a fruit. (Most all of what we refer to as cereal grains in laymen's terms are caryopsis. So, your friends assertion that it is a starchy grain would be correct from a botanical point of view. And nutritionally speaking the difference between starch and sugar is in flavor. They are both carbohydrates. So it hardly matters from a dietetic point of view whether you call it a grain or a vegetable - it is still high in carbohydrates

The US Dept of agriculture's food pyramid would place sweet corn in the vegetable food group, however. Not in the grains group with field corn products like corn flakes or tortillas. So if you are using the food pyramid to determine how many portions to consume, you could count yellow corn as a vegetable.

Corn is a grain because it grows on the stalks like oats and wheat.

its classed as a veggie to the US goverment

however the high sugar content, and minimal (though existant) vitamin/mineral content, most people consider it a starch, similar to bread, potatos and other high

its technically a grain.