Workplaces aren't like any other place; if you have a nasty fallout with a friend from the gym, you can easily switch gyms. If you have a fallout with one of your bosses or coworkers, it's much harder to avoid them, and the fight comes with the anxiety of your income being at risk; ultimately, your survival ends up entangled in the work drama.
When we have a rigid belief system, and something happens in our life that reinforces those beliefs, we tend to write it off as the ultimate proof we needed to justify the way we're living.
I personally don't believe relationships in life should be so formulaic. A lot of people (myself included) have found amazing, lifelong friendships at the workplace.
You should definitely learn to spot behavioral red flags in people, in any place, not just at work, but that doesn't mean distrusting everyone; there are good people out there!
After reading this story, I was left with the impression that this employee already had trust issues, and this particular situation she went through only confirmed her bias, but the conclusion she drew from it is not the only one you can get to, in my opinion.
Maybe this experience isn't telling her to stop trusting coworkers, but telling her to stop trusting shady people.