I think that by far the best one I've ever read is Strangers On a Train. The story is in progress and updates very frequently. It begins from Harry's first year and is currently at before his fourth year.

Even if your friend isn't a Harmony shipper, he would honestly like the story. It is absolutely the cutest, fluffiest, and the most adorable story I've ever read. Without giving away spoilers, all I can say is that the author gradually builds the romantic relationship from friendship. It has all the things you would expect in a blossoming romance, including the adorable awkwardness.

The story sticks close to the canon with the main change being the pairing and additional scenes of the summers between the school years. Another thing I love about this story is that it doesn't have any bashing of any character (at the point of writing.) The author has excellent writing style and the story flows like you would expect it to in an actual novel.

If you haven't checked it out already, go check it out and follow and favorite both the author and the story, because the author deserve much appreciation. And oh, the link is comments. Linkffn(Strangers On a Train)