The friend from the window next door

Since you were little, the window of your bedroom has looked out onto another bedroom, that of your neighbor and what has become your best friend. So you grew up together, spending long afternoons at his house or yours, talking for hours in the evening through your windows (before your parents came to scold you). However, one day, on a morning that seemed like any other, something happened: -Your neighbour/best friend wants to tell you something. -You see something you shouldn't have seen. -Your neighbour/best friend sees something he shouldn't have seen.