In addition, Musson was accused of saying that "the thing [JP] needed was a f**king hiding and he would be the one to sort [JP] out."
RHHL appointed its trade and general manager Grant McLeod as its decision-maker for the disciplinary inquiry.
At a disciplinary meeting held on November 23, 2017, Musson was asked to explain what happened, the authority heard.
According to notes made from the meeting, Musson said the background was "a relatively big incident between me [Musson] and [JP] in the past that never got sorted out."
The authority heard Musson told those at the disciplinary meeting that he considered what he said to RH was "a private conversation" but accepted other workers were within hearing distance.
Musson denied saying JP "needed … a hiding", but told the authority "what I think I said was that someone should give him a hiding, not that I should be one to give him one".
Musson also denied saying he would punch JP's face or "knock his … lights out".
Four days after the disciplinary meeting they met again where McLeod delivered his decision to dismiss Musson.
Gibson had asked McLeod to consider imposing a final written warning rather than dismissal, noting that Musson had been allowed to keep working during the course of the disciplinary meeting, which suggested there was still a level of trust and confidence in him.
In its determination, the authority ruled RHHL's decision to dismiss Musson for serious misconduct was faulty and its actions unjustified after their own investigation found RHHL had failed to conduct a sufficiently full and fair investigation of its concerns about Musson.
Robin Arthur, a member of the ERA, said RHHL could not safely reach its conclusion of serious misconduct given the shortcomings of McLeod's investigation.
The authority found three key failures into the failure to conduct a full and fair investigation were the decisions or oversight made by RHHL in not interviewing fellow workers RH and BC or Musson's former department manager Ray Archer, who were each likely, in the authority's view, to have relevant information that needed to be gathered.
It further added it was unsatisfactory to simply accept RH saying she did not want to comment or be involved.
Arthur said the defects in RHHL's process were more than minor, and Musson was treated unfairly because relevant information was not gathered, checked or provided to him for further comment or explanation.
RHHL managing director Cam Caithness said the ERA's decision was based around the fact that a team member wasn't made to give a statement.
"We asked her to make a statement, she was in fear of repercussions … and we had to take her well-being into account."
After awarding Musson $10,000 compensation for hurt and humiliation, the authority applied a one quarter reduction due to elements of Musson's conduct contributing in a blameworthy way to the situation giving rise to his grievance.