Well, the manuscript you might present can depend on the school and what genre you're applying to: fiction, non-fiction, poetry, and writing for children (these are the most common available.) Assuming though, as you mentioned fiction, that he himself is interested in fiction, I would suggest either to submit two to three short stories (his absolute strongest work) or an excerpt from a novel. You can also try a combination of short story and novel. However, I should warn, most schools like to see three short stories or less. Submitting more than three short stories in a manuscript is generally looked upon badly. I assume this is because most manuscripts average about 30,000 to 40,000 words. Several universities recommend not using short stories under 10,000 words. This is probably because anything shorter can't reveal enough to the universities about one's writing ability. Also, the university needs to see one can write for both quality and length. Always be sure to read specific guidelines from the individual university though regarding manuscripts.
As to what specifically to submit for stories, there is no right or wrong. Submit the absolutes best work available. I've heard of submitting genre fiction to personal narratives. I personally submitted two personal narratives and one fictional mystery piece to most the universities which I applied to. To a few schools though, I submitted a personal narrative and an excerpt from young adult novel. Never submit writing geared towards children or young adults unless it's to a writing for children program such as at the New School or Chatham University.