I agree with Daneen's response but I don't think that's all there is.
I think absolutely asking for communication in writing is one point.
Second point is documenting for themselves the times abusive CW has approached them and as many direct quotes as possible, including date and time (creating evidence)
Third point, if this occurs in the presence of others (open plan office environment) is to state firmly and clearly "do not speak to me like that" and walk away
Fourth point, talking to a medical professional about the issues and getting medical certificates if days off are to be taken - again creating a paper trail and also preventing the Op from being blamed for being unreliable etc and perhaps getting a warning, or their job being at risk.
Fifth, when Manager says "you have to work harder to get along" present manager with all dates/times/evidence/quotes and formally request action be taken. If manager says no, take it to HR, higher manager, CEO, or even Board if there is one. Preferably in email creating a paper trail.
Finally, I don't know how industrial relations legislation works where this letter came from, but having a basic understanding of Australian IR/HR legislation, this behaviour is bullying and the company could be liable if the are made aware of an issue and refuse to take action (even offering mediation if they really believe there are 2 sides to every story) and could face hefty compensation claims and significant risk to reputation