The Public's business

Can a person in public office legitimately use his position for private gain? This question is couched in the language of a teenager as we sit on a committee of high school students planning refreshments for a forthcoming sports day. One boy, employed part-time by his caterer sells them on the idea of having his boss provide the eats. He later brags to a friend that he stands to profit on the deal. His friend objects on ethical grounds. Is he right in his objection?