"Traitor" is a pretty powerful word. A more realistic way to look at it is that someone—using a persistent method of communication—told him that they had committed a very serious crime¹, and he then had to choose between covering it up or going to the police. Covering it up or ignoring it could have meant he was an accessory.
¹ Whether it should be a serious crime or not is irrelevant here.
Under US law, most people in most situations are not obligated to report crimes they become aware of.
Accomplice liability only comes into play when a person intentionally, actively aids a person to commit, conceal or profit from a crime, or to escape prosecution. Inaction doesn't count.
And to be honest I'd rather he snitch than contribute to a culture of cronyism that's all too prevalent in the world.
Like, discovering that a friend of yours committed a serious crime and then keeping it a secret is the very definition of corruption. If we, as a society, are going to move past corruption and cronyism, then we have to accept this kind of snitching, because it's the only way to defeat corruption.
For that matter, do you want to end a culture of people being sexually assaulted at frat parties in full view of dozens of people because none of the witnesses will go to the authorities? Then normalize snitching.
Yeah, best way to end corruption certainly is to help those in power keep their secrets by snitching on those who tell the world, that makes sense! After all, those in power have said it's a crime to challenge their power!
¹ Whether it should be a serious crime or not is irrelevant here.