I don't think anyone could make a sound legal case for 'harm', though.
The film distributors would presumably sue the film's producers (who apparently did their own due diligence) rather than the publishers, and honestly, anyone genuinely coming off Pregabalin and going on a 600 mile walk because someone in a book (even in a book classified as 'non-fiction') did so with good results is a bit mad, and I think would be likely to be destroyed by the defence if they brought a case.
And TSP itself (though not SW's claims in interviews or the sequels, as we've often said on here) is actually pretty careful not to make any medical claims that could be viewed as anything other than the subjective account of a non-medic catastrophising and then desperate to see some improvement in her husband, and makes it clear the diagnosis is tentative at best.
I think a more likely money-loss scenario would be PRH deciding to cancel OWH and requiring SW to repay her advance, but even then, PRH will also have contractual obligations outlining the situations in which they can not publish the contracted book.
If PRH outline sound commercial reasons for not publishing it (SW's damaged reputation meaning low sales etc), a judge might well find that the publisher should not be forced to make a financial loss and find in their favour, if SW sued for loss of earnings.
But rather than going to court, PRH might decide to return the rights to the author, saying that SW has suffered no financial loss and that if the book is good enough, the author will have no difficulty finding a publisher elsewhere.
If no other publisher is interested, then PRH are right in thinking the book wasn't commercially viable.
To counter this, SW would then have to prove that her inability to find another publisher was caused by PRH's breach, that their failure to publish was widely known and that it damaged the book's prospects elsewhere etc.
A bit of a quagmire, to put it mildly.