Nick walking in on you has a fairly low random chance to fire, so it can take months of in-game time to grind that one up, especially since there's only a random chance that he will even be in the house.
People get hung up on this because they do what seems like the right thing (be in the bathroom at 6AM in the morning) for a bunch of times and don't get the event so they think they're doing something wrong but it's just because the random check is set low so you have to try over and over until it triggers. It's basically a repeat of the "get blackmail photos" quest from Big Brother, except that time you only needed to be successful once.