I read up a bit more about loading and unloading because I clearly am missing some crucial information here as well. I don't use scenes though, only prefabs. I wonder now what happens once you load them into memory and then close the game. Will it be in memory until the OS unloads by force?
And I also read that in fact all game objects, materials, textures etc are being load into memory as soon as you use them (for example at scene loading). So it's not a matter of using load actions or not. Loading only is a way to, well, load them at specific moments where it's convenient for you (or not load them at all). But once you loaded them automatically or through a load action, it will stay there in memory. And I guess, it's Unity that decides to get rid of it through garbage collection (correct me here if I'm wrong) whenever it's necessary.
Unloading (the action) seems only will work if you first get rid of all references in the game. Meaning even if you have a reference somewhere else to your scene, it won't be able to unload it. Same for materials, textures, etc So in most cases, unload won't do anything.
In this regard, to your problem, I am with djaydino here, I don't think that your game was capable to 'destroy' your friends computer in this way. The only possible thing that could have happened is that your game filled up the memory (like Internet browsers do for example, just way quicker) and that this ultimately lead to a crash. If with this crash the harddrive got damaged it's more so that the harddrive already had a problem and by sheer bad luck it happened at that very moment.
