Well we both know that the response is almost going to be that The New Guy is too big for his britches and should shut up. How dare he tell us how to code!
Bad practices aren’t always just differences of opinion; I saw one company, Midisoft, go out of business because the CEO/CTO wouldn’t listen. The night before a big investor meeting he sat in a hotel at his laptop frantically trying to get the product to work; it was unsalvageable. It was my work, while he was preoccupied with another project, that got it on the market at all.
And he resisted every step of the way.