I'm sure we've all worked with a person in the past who doesn't know how to be a coworker. See, when you work in an office, slowly and over time, you'll learn the ethics and the unspoken codes there are. And when it comes to coworkers, this is especially important, since you are an equal, and being a good peer and a sort of "backup" is important for having a good relationship with them, as well as a healthy relationship with work and the office.
That being said… I think this man was never told about these things.
When someone takes their lunch break, which is very important for mental health and for taking the time to recharge and to relax a bit before getting back to things, the other coworker should cover for you, because they will do the same thing when you're on your break. Is that simple… And, according to the story posted in r/Career_Advice, I guess the man was upset that she didn't want to take that client call and decided to roast them in front of the client, telling him lies about the employee.
I'm guessing they work in a sales department of some sort, so having a strong and good first approach with a potential client is fundamental to success in the business, so you can imagine that telling one that the employee "can't remember them" is just a big no-no. And I'm sure the man did this out of revenge, because the employee didn't want to ruin their unpaid 45-minute break with a call that could have perfectly waited a couple more minutes.
Again, this is a good example of how not to behave as a coworker. I just don't understand why someone wouldn't try to be a good friend in this case and respond politely to the client that they are not available at the moment and will reach out in a moment… it was that simple! Even I, who'm not working in any sales department, was able to answer that in a split second.