I say even if you’re writing fanfiction, you should try your absolute best. Writing isn’t so much about talent as it is about hard work and persistance. Furthermore, writing is meant to entertain people. Clichés aren’t entertaining. They’re boring. If you’ve seen something fifty million times before, then there’s no reason to read it again in sEpHiRoThD00dZ’ work on Cloud and Red XIII’s forbidden love.
Also, don’t discourage him from reading the greats! I think he should read some Dashiel Hammett for good measure. If you like fantasy, the Dresden Files are a suprisingly good fantasy/hardboiled detective mix. I highly recommend them (start with Storm Front).
I would argue that a story doesn’t need to be understandable to be good, and that it doesn’t necessarily need good grammar. But you need to have a good grasp of grammar and story structure in order to pull that off elegantly. But I guess I’m getting a bit ahead of myself. I’d say the only things a story needs are a beginning, middle, and end.
If you’re serious about writing, I’d look through this site here. It’s about science fiction, but it translates well to other genre fiction. It’s written mostly with teenage/young adult audiences in mind (though it’s easily used by adults as well), but a lot of the stuff still holds true.
http://www.writesf.com/ <—Seriously, check it out.
For more general writing, Stephen King’s On Writing is an entertaining book in its own right, and you learn a lot about his life as well. Don’t like horror? That’s fine. The book doesn’t have anything to do with horror. It’s also part memoir, and his life is pretty fascinating.
In any case: KEEP WRITING. It’s the only way you’re going to improve. I’m not trying to discourage you in the least. Nobody starts out as a naturally talented writer (unless you believe Heinlein).
I have to address this, though:
“One man’s cliche is another’s classic literary element (and vice versa.) ;)”
Not really. There’s a huge difference between tropes/archetypes/literary elements and clichés. Literary elements are generally unobtrusive and often unavoidable. There’s a difference between having a king that travels disguised as a common man and copy/pasting Aragorn. 