A Human Resources professional once told me that there was no need for managers and supervisors to get assistance on management questions from any other source since all they had to do was to pick up the phone and talk to HR.
I wondered what planet he'd come from.
What if, I diplomatically responded, the manager or supervisor is uncomfortable surfacing a question with HR?
He assured that that they were quite comfortable doing so. That opinion was not shared by the people I'd met with in his organization's other departments.
Now you can say that a management consultant has a vested interest in promoting external consultation and that an HR professional has one in restricting the sources of management advice. That doesn't mean that one side or the other isn't correct.
The point that I believe tips the scale in favor of opening the doors and permitting managers and supervisors to get advice from a variety of sources is to consider what will happen if they are not permitted to do so. They won't go to HR. They'll rely upon their own analysis and, in many cases, will postpone taking action until the problem becomes much worse. HR will eventually see the problem on its desk but only after months of omitted or poorly crafted action.
A Human Resources department that is top notch doesn't need to discourage employees from seeking outside advice and an HR department that is not top notch doesn't deserve that restriction.