In an interview with Deirdre Bolton on Fox Business News on August 7, 2015, Ralph Nader said he was “delighted” to learn that Donald Trump was entering the presidential race, possibly as a third-party candidate:

The two party tyranny that blocks voter choices and dominates the political scene on behalf of big business needs to be broken up and Trump is the one to do it... He is with the progressives on challenging the rigged trade agreements… He’s going to be a very interesting factor: the nightmare of the corporate Republican establishment, and it’s a breath of fresh air. He’s also gaffe-proof, they can’t trap him. He just turns it right around on the reporters who question him. By being brazen, Trump is punctuating the progressive agenda — the progressive critique of big business, Wall Street over Main Street. ... I’ll tell you, I’ve always said the only people who can break up a two party tyranny that stifles the voices of small parties and independent candidates are billionaires, and we got one called Trump.

Bolton asked him if all this means he’d consider voting for Trump:

No, because he stands for a lot of things I oppose; however, what he’s doing, he’s breaking up these sloganeering politicians.

After Trump became the leading Republican primary candidate, Nader’s delight lessened, complaining in a May 13, 2016, interview that Trump “lowered the level of political debate.” But he still preferred Trump as president to Hillary Clinton:

But in an interview with U.S. News, Nader expressed more positive thoughts about Trump’s candidacy than Clinton’s. The liberal activist says Trump has brought some important issues to the fore. “He’s questioned the trade agreements. He’s done some challenging of Wall Street – I don’t know how authentic that is. He said he’s against the carried interest racket, for hedge funds. He’s funded himself and therefore attacked special interest money, which is very important,” Nader says… When asked what positive contributions Clinton has made to the 2016 campaign, Nader called her a “corporatist, militarist Democrat… She’s going to win by dictatorship.”

Similarly, during Nader’s 2000 third-party presidential run, he often said he wanted George W. Bush to beat Al Gore.

In the May 2016 interview, Nader expressed admiration for Bernie Sanders, especially for taking down Clinton:

he says Sanders' candidacy was meaningful because it simultaneously pressured and exposed Clinton.

So... will Nader ever express regret for his praise of Trump? No matter how much irreversible damage Trump does to this country, to people’s health, to regulatory agencies, to the planet? Given that he never expressed regret for saying George W. Bush would be better in the White House and on the environment than Al Fucking Gore, I’m going to go out on a limb here and say no.