Shorter sentences work better. In the 'American method', at least. Albert Camus wrote the first half of The Stranger this way--precise and briefly. You can see how in the second half of the book, he changes up the style again to better illustrate the inner world our protagonist has retreated into. But it's important to change things up in your writing. If you keep things running on too many times in a row, you'll lose the reader. Each paragraph has its own cadence. Longer sentences aren't any smarter or more adult than short ones are. Probably less so. When I write, sentences always goes from short to long. Then from long to short. And once I'm sure the reader is well-rested I can hit them with a sentence that stretches way out passed its due date, and maybe it even continues with a comma splice. Tell him/her to read their work out loud. The awkward and breathless comma splices will make themselves much more apparent.