My friend has sent me a work to critique which he hopes to make into a light novel. I've never heard of this type of story before and a quick Google didn't give me much context about how it differs from a traditional western young-adult novella?
I don't know about Sweet72's reference to Japanese literature or how that relates to English literary forms.
But in common American usage, a "novella" is a shorter work than a "novel". Some give formal definitions, like a novel is over 40,000 words while a novella is 20,000 to 40,000. But I think the real, practical definition is that a novel is a story that takes a full book, while a novella is a story of such a length that you would normally put 2 to 4 together to make a book.
Calling a story a "light novel" has nothing to do with length. Rather, that means that it is easy to read, fun and escapist, like an adventure story or a romance. This is as opposed to a "serious novel", which has deeper themes and embodies commentary on society or history or the nature of humanity. For example, "Atlas Shrugged" would be considered a serious novel, because the whole point of the book is to discuss the author's ideas about politics and economics. Any of the James Bond novels would be considered "light" because they are about adventure and romance and make no attempt at profound themes.
Of course any such distinction is vague and debatable. A book could include serious commentary about life while also being easy and fun to read. I've read plenty of books that are mostly adventure stories but that also include themes about the human condition. Personally I think a lot of the better science fiction falls into this category.
Either a light novel or a serious novel could be in pretty much any genre. You could write a light Western novel or a serious Western novel, etc.
As per Wikipedia's Light Novel article:
A novella is a short story and generally features fewer conflicts than a novel, yet more complicated ones than a short story.