So does the author think that everyone is great at their job? What do you do when you have an employee that can't do the work and you are confident that they won't improve?
Pointing out the age difference in the manager and the employee, and their sexes, is just baiting. Unless he wants to make the case that there was sexism or agism from the manager, or that the company engaged in sexism or agism, it is just a red herring. In fact, it's quite cowardly, since it implies something the author isn't willing to actually say.
Finally, the fact that the person who was fired was with the company for 4 years doesn't mean anything at all unless we also get the context. It may be that this person struggled from the beginning, and they hesitated and didn't fire her for four long years. Or it may be that they hired her to do some specific job, and that job went away, and despite repeated attempts she just could not manage to learn anything else.
The thesis that she was 'disposable' is just ludicrous. If she was good at her job, then the manager is just an idiot who is cutting his and the companies own throat, and we can't make a moral argument against stupidity. If she couldn't do the job then there is no reason to be discussing this. If she was borderline, we still can't make a moral case, since borderline situations are at worst slightly wrong.
So once you cut through the left handed implications, I don't think there is any argument left.