After careful consideration, Snape still decided to tell Dumbledore the news that Narcissa intended to sneak into Hogwarts to steal Felix Felicis. Since that elderly man was willing to give Draco a helping hand, he could certainly handle this matter properly as well. He likewise did not want Narcissa's actions to cause trouble for Dumbledore's plans.
Although Snape had never used Felix Felicis himself, as a potion-maker, he was well aware of how miraculous this potion was. Once Draco used Felix Felicis, he might actually succeed in repairing the damaged Vanishing Cabinet ahead of time.
Once the Vanishing Cabinet was completely repaired, the plan to invade Hogwarts would be brought forward and become uncontrollable, which would likely disrupt Dumbledore's original schedule—hardly good news.
Chapter 1167