People disappoint us. I know this employee would've benefited from acting more maturely in this situation, but that wouldn't have changed the fact that her work bestie betrayed her.
I really don't think the solution to this type of issue is to stop trusting people. At some point, we should accept that things like this happen. If we walked around afraid that somebody will eventually betray us, we wouldn't be able to trust anybody, make friends, or fall in love- anything.
To make friends, you have to trust. You can be cautious, wait longer, whatever, but at some point you have to trust. The same goes for romance. We can be cautious all we want, but some people who will betray us won't tell us in advance through their demeanor.
Bad things happen, and we shouldn't generalize that and stop trusting altogether. We have to stop trusting only the ones who betrayed our trust and move on tomorrow, befriending better people; that's it.
This employee was shady for not confronting the guy and getting carried away by her anger, but well, sometimes our emotions get the best of us. I think if she had confronted him, she would've found out sooner that her bestie wasn't as great as she seemed. Two-faced people do exist! Sadly.
It might be hard to recover from the blow because the situation mattered to her, and to keep seeing the one who betrayed you every day is far from ideal, but time heals all wounds.
Maybe she can start plotting a very elegant and elaborate revenge. That would be interesting. I can't think of one, honestly. Should she confront her? Should she start another rumor about her? Should she leave everything in the past and be the bigger person? Who's to say?
What would you do in a situation like this? Please don't say find another job.