Two things will help: writing and reading.
Your friend should start writing about whatever comes to mind, and do that on a regular basis. The writing should remain private, to alleviate the anxiety that might arise if he anticipates that someone might read it later. Start with a pencil and paper - or a book. I suggest a pencil because it's slower than a pen and needs a level of attention that writing with a pen does not need. Slow down, consider your thoughts, and then take the time to actually write them out by hand goes a long way.
Read a lot too: news, magazines, books (modern and the classics) - get a feel for what you like and, more importantly, why you like it. Write about what you read. I usually have my writing book next to the book I'm reading and have a written conversation with the author, the characters, or simply just write about whatever I'm reading.
Later, once your friend is more comfortable writing, have him write articles like opinions, reviews, a short story, or even directions for using a coffee maker - something that he knows someone else will read and edit. And, like it says on the bottle: later, rinse, repeat!