Honesty is conceptually the best policy, but in real life it can be a lot trickier than it sounds. For instance, if you know about a coworker's unsavory behavior but don't have a close relationship with their wife, is it your responsibility to tell her, or should you abstain from narcing?
When it comes to the concept of "telling" on someone, situations can quickly get dicey and there is rarely a clear answer. Save instances of clear abuse or danger, getting in the middle of other people's business, even if well-intentioned, can often backfire.
"AITA for leaving a co-worker stranded in Detroit, and not telling his wife the truth about it?
I've been giving a co-worker of mine a ride back and forth to work for the last couple of weeks now since his car broke down. He only lives a couple blocks away from me, so it's not like I'm going out of my way or anything."