In the UK employers have a legal duty of care to protect their employees from harm in the workplace, and this includes stress.

This is written in law, and has been supported by court cases.

One incident like that is wrong, but could perhaps be explained by someone having a really bad day. Recognition, apology, and no further incidents would help.

But continued incidents? The company is leaving themselves open to lawsuits. ("Constructive dismissal" and employment tribunal in the UK.) I'm pretty sure that toxic work environment leading to poor health is something that has been through US courts.

I mention this because sometimes the only way you get through to PHBs is to talk about costs and risks, and not "don't let employees be dicks to other employees".