I start by evaluating whether it’s 1) within my job responsibilities to be involved and 2) I feel personal obligation.

As you’ve described the situation, and also your personality with a strong avoidance of conflict, if I were in your shoes I might support my colleague by getting the person who is supposed to intervene. Maybe that’s a manager or security if there’s signs it might escalate to violence.

I’m not a bouncer, I’m not a security guard. I have no job obligation and no personal obligation to cut into a violent interaction. I can be more helpful by getting someone trained for and paid to deal with the situation.

If I’m the manager, I have more responsibility to intervene. If the person getting yelled at by the Karen is like, a family member or a close friend, I would be more likely to intervene. Work colleague does not invoke the same personal responsibility I feel towards friends and family. I would also not say this to my colleague in the way I’ve described in my paragraph.