Some remote teams are like co-located teams and work synchronously with set office hours. Other remote teams work asynchronously instead and individuals are free to arrange their work and life as they see fit. In the latter case, if somebody is working on a task that'll take a few days, not hearing from them for a day or two isn't unusual or a problem.
It sounds like perhaps he thought all remote work was asynchronous, but his employer wanted him to work synchronously. But it would take a severe communications breakdown for that to get to the point where a weekly performance review was needed. Normally, you'd talk about how this all works before the person is even hired.
But honestly, when he describes it as humiliating, having a problem with being managed by younger people, and characterising being "judged" by his bosses as threatening, it does give the impression that the prima donna label isn't far wide of the mark. It's absolutely fine for younger people to manage older people, it's not humiliation to have weekly meetings to resolve a performance problem, and your bosses judging your performance is not a threat, it's a mandatory part of their role.
Frankly, I'm not surprised. Often these expectations are not explicitly set. And often (probably more often than not), managers will let it get to the point of "exploding" and suddenly you find yourself in such a meeting. Many managers are uncomfortable with these conversations. They are often feeling defensive going in, and to counter that they will wait until they have enough "data" to have the conversation. But by that point, the damage is done.
Severe communications breakdown? Perhaps. Rare? Not at all.
If it was his non availability when expected (meetings/telecons) or missing explicitly agreed to deadlines, I agree with your non-professional label.
If he got a task to do in 3 days and delivered in 3 days he did just fine even if he was on a beach with the phone off the first day. In this case the failure is with the management who didn't know how to run a remote team (which is a rare skill, too).
If you aren't using a vacation day then you should absolutely be reachable throughout the entire work day, and disappearing for a day at a time is unacceptable.
This is possibly indicated by "bosses". Not having explicit line management sucks.
Works best if that is early on after joining the company. Or just ask the same question to HR, they would guaranteed know what the process is.
There might not be a clear vacation policy (as much as you like etc).
Ask HR? I've worked at companies with 100 people who had no dedicated HR staff...
When you can turn to your coworker next to you and ask a simple one minute question that unblocks you, but you can't reach your remote coworker for an entire day and you thus fail to make progress for an entire day because of their truancy, you begin to resent them really quickly. It doesn't matter if you're their manager or not; it's super irritating.
Part of what makes corporate America suck is the kind of passive aggressive resentments that never get expressed. Suddenly the remote worker finds that everyone hates him and has no clue why.
A contract is an agreement. There are companies paying very high salaries to very low - expecting very high commitment to very low - in various combinations.
It's up to the company to clarify expectations.