I think this is 100 percent know your audience. I've been in Toots' position before, where I didn't care about evals or what was said, and didn't have time to worry about it. I've also been in the complete opposite situation, where talking about evals was a hangin' offense, and I needed my best employee (and friend, which may or may not matter) to keep a pulse on these things. If someone was committing what would have been, in this case, a major violation of policy, of course I'd want to be told, even if I chose not to act on it right off.
I think a lot depends on how big a violation this is, because this seems to vary. Most important, though, is the the employee/boss relationship, and whether the boss relies on the employee for observing things like this. In that case, I don't think it has to impact the employee directly - her loyalty is to the boss.