There are few more perilous lines of work than being an ally of Donald Trump. Vouch for him one minute, usually by insisting that the latest accusation against him is bogus, and the next you’ll be left looking like a fool – as he or his family confirm that the very charge you dismissed as fake is, in fact, true.

It’s happened again, this time via the Fredo Corleone of the Trump saga, Donald Trump Jr.

White House chief of staff Reince Priebus dutifully went on TV on Sunday calling Donny’s meeting with a Russian lawyer “a big nothing-burger”, a harmless chat about adoption. But then along came the boy prince himself, releasing an email chain showing that in fact he took the meeting in anticipation of receiving dirt on Hillary Clinton, and on the explicit understanding that this was part of a Russian government effort to tilt the election towards his father. Rather a lot of meat in that nothing-burger.

So, as Priebus and dozens of others in Trumpworld have learned, there’s danger in defending these discount-store Corleones, whose shadiness is matched only by their gormless ineptitude. (Imagine the IQ of Trump Jr, who not only handled this negotiation with the Russian lawyer by email but declared in writing, of the offer of intel on Clinton: “If it’s what you say, I love it.”)

No wonder Susan Hennessey, a former attorney in the National Security Agency, tweeted this advice on Tuesday: “I cannot say this strongly or sincerely enough. If you work in the White House, you need to have a plan in place for retaining a lawyer.”

But there is one ally of the US president whom no lawyer can help. Her problems run far too deep for that. Her name is Theresa May.

She tied herself to Trump when he had been president for a single week, rushing to Washington to win the race to be his first foreign visitor. She held his hand and offered him the shiniest bauble in the UK prime minister’s gift bag: an invitation for a state visit. While Trump’s predecessors had had to wait years for the offer of a royal red carpet (rather than just a regular working trip), and some never got one at all, May bowed early.

That looked embarrassingly eager at the time, especially when, just a few hours after he had stood with May, Trump turned himself into a global pariah with his travel ban targeting seven mainly Muslim countries. Angela Merkel had made future ties conditional on Trump’s adherence to basic international norms, such as human rights. May, by contrast, was supine in her neediness.

And that mortifying posture has continued. When Germany, France and Italy issued a joint condemnation of Trump’s break from the Paris agreement on climate change, May pointedly refused to sign. The PM promised instead that she would raise the subject when she sat down for formal talks with Trump at last week’s G20 meeting in Hamburg – only to admit afterwards that she had done no such thing. There wasn’t enough time, Downing Street said, even though the Trump-May session overran by 20 minutes. (Officials said the pair discussed the issue informally, after the meeting.)

It was also at the G20 that May once again stood at Trump’s metaphorical side, defending his decision to have his daughter Ivanka take the US seat at the talks, putting this unelected designer of handbags between May and President Xi of China. To most observers that looked like an act of regal presumption, Trump confirming that he sees the US presidency as a throne stamped “Property of the Trump Family”. Not May, though: she thought it “entirely reasonable”.

Received wisdom says May has no choice – that cravenness to Trump is the necessary consequence of Brexit: we need to grovel if we are to get that all-important trade deal. It is quite true that one of the multiple absurdities of turning our back on our nearest neighbours is an increased reliance on those much further away. Even so, that did not compel May to hitch her wagon to a Trump star that was always likely to explode.

Given how long such agreements take to broker and seal, she could have made clear that Britain’s relationship is with the US in its broadest sense, cultivating ties with the Congress, which will have to ratify any final deal, rather than with a president whose longevity was open to question from the very start.

We don’t yet know whether Don Jr’s emails will prove to be the smoking gun in this affair; worse may be yet to come. But the prudent approach, clear even before Trump took office, was to wait and see, rather than rushing headlong to be the new and volatile president’s best foreign friend.

In an admittedly crowded field, it stands out as one of Theresa May’s most severe misjudgments. Already reduced by Brexit and her failed election, this diminishes May further – and it diminishes Britain too.