Your example is exactly the same problem as "admissible evidence" in a court of law. In the USA, it's very common for evidence to be rejected because the collection of that evidence was itself illegal - this is intended to protect the integrity of the system in general, no matter how heinous the alleged crime in a specific case.

So I'm sceptical: was the union really defending that specific employee, or were they trying to prevent a precedent from being set that could be used against other, more upright employees?