When challenging a bully watch for this fifteen-step behaviour exhibition:
6.Threat:
Put the frightners on. The victim has always succumbed before, see if they are still susceptible.
This is doing your reputation no good.
I'll have to bring this up with the Sector or Area Manager.
I'll have to consult with our/my/your professional body on this.
I've talked with [a certain person/office] and made them aware of the facts of this case.
If you persist in this ridiculous campaign, I shall have to report you to Board of Directors/Head of Personnel/Security/Police.
Note that most of these threats are implied rather than real. In most cases, the bully has no intention of contacting anyone, but the purpose of making threats is to stir up fear and confusion. An enemy confused is an enemy disheartened is an enemy crushed. In a few cases however, the bully may carry out the threat - and follow it up with uncharacteristically meticulous notes copied with equally uncharacteristic rigorousness to all and sundry - or at least all whom the bully regards as allies or potential allies.
A further effect of this approach is to imply that the bully has the full support and backing of the people they name. Again, this may be true. It is also possible that at this moment the bully does not have the support of anyone, but in the event of a showdown these people may side with the bully, thus turning the bully's threat into an eerily self-fulfilling prophecy.
Threats themself are provocative, anticipating the next step. If you suspect the bully really has consulted a third person, and made them aware of the "facts", pause and consider how to approach the situation. A sudden reaction, which is what the bully wants, may be inappropriate or unnecessary, and may make the situation worse. A considered, discrete enquiry at a suitable time may be more appropriate. If you do decide to double-check, remember that opinions can be challenged, altered, misunderstood and misrepresented - so stick firmly to facts.