Disclaimer: IANAL
I'm sorry to say that, but your co-worker is in a very bad situation. What they're trying to do is to fire and document it properly so that he can't sue them. His past past accomplishments and tenure with the company are irrelevant. They are trying to throw him away like a used rubber.
The so called performance improvement plan (PIP) is a sham in most cases. It is not about improving performance. It's just a way to get rid of an employee without paying severance or getting sued. His actual performance doesn't matter. They will always find something to write him up for even if he does his best.
Here is what I would do:
-Don't argue or openly confront anybody at work, just do the bare minimum to not piss the management off.
-He should find out exactly how long the PIP is for (e.g. 2 months) and treat it as his notice period to find a new job. However, if he doesn't comply (e.g. skips work for no reason, plainly refuses to do what he's told, etc), they can fire him even before the end of his PIP.
-Read all HR handbooks and policies, particularly regarding PIP and termination. What happens to his pension (if any), accrued vacation and sick leave? What are the rules regarding warnings and feedback for poor performance? For example, if his manager was supposed to give him a warning prior to starting PIP but didn't, your colleague may be able to use it against the company.
-HR or whoever performs HR function at the company is not on his side, but if his manager breaks the rules, they may step in to cover the company's ass. Depending on the situation, this can work in your colleague's favor.
-If he has enough money to retire, maybe it's his opportunity to do so. Otherwise he should start actively looking for work, though at age 58 it's going to be very difficult. If he has to look for a new job, he should put all his effort into it rather than fighting the current employer.
-If he decides to fight, he should discreetly talk to other colleagues who were let go recently or are in a similar situation and see if he can pull resources with them. Maybe one of them knows a good lawyer who helped them get severance, etc. Maybe they can look into filing a class action suit, though most companies these days make their employees waive their rights to sue collectively. Normally, this waiver is in the offer letter or other documents you have to sign before you start working for a new company. Whether or not it's enforceable, I don't know. That's something a good lawyer should be able to tell.
-He can talk to a lawyer, of course, but only if he wants to fight the company. If he has a legitimate case, most likely it will be resolved by mediation, not in court. I wouldn't bet on it, though, unless the lawyer can prove clear pattern of discrimination against older employees or any other protected class.
Also, like other posters mentioned, you should start looking for a new job too. Here's a little poem about fascists for you to consider.
First they came for the Socialists, and I did not speak out—
Because I was not a Socialist.
Then they came for the Trade Unionists, and I did not speak out—
Because I was not a Trade Unionist.
Then they came for the Jews, and I did not speak out—
Because I was not a Jew.
Then they came for me—and there was no one left to speak for me.