The lawyer representing the Meta whistleblower Sarah Wynn-Williams has said he too has been prevented from promoting her memoir under a legal ruling, after her silent appearance at the Hay festival.

Ravi Naik said the terms of an arbitration proceeding meant neither Wynn-Williams nor her “agents” could promote her bestselling book Careless People or say anything disparaging about the company.

Naik spoke after Wynn-Williams was forced to sit in silence during an appearance at Hay on Sunday owing to the terms of the ruling. Naik said an interim arbitration ruling meant she risked being forced to pay “punitive” damages if he promoted the book.

“Never in my life have I faced a circumstance where my client cannot speak about her truth and I as a lawyer cannot speak on behalf of my client,” he told BBC Radio’s Today programme on Monday.

Meta has claimed the book, which made a series of claims about the social media company’s behaviour and culture, is false and defamatory. It also contained allegations of sexual harassment that were denied by the company. Meta says Wynn-Williams was fired for “poor performance and toxic behaviour”.

Naik, legal director at law firm AWO, said Meta’s stance on Wynn-Williams’s Hay appearance was not a “hypothetical threat”.

Meta had said in writing that they considered Wynn-Williams’s attendance at the Hay talk would be a “breach” of the interim arbitration award, according to Naik, and they would seek sanctions if she promoted the book or criticised Meta in her appearance. Naik said Meta would probably seek to uphold the arbitration award, handed down in California, through the British courts.

Wynn-Williams was due to appear on stage in conversation with the investigative journalist Carole Cadwalladr and the academic Tim Wu but spent the scheduled hour sitting in front of the audience without speaking. She was also unable to nod or shake her head.

Introducing the panel, Cadwalladr said: “I think this might be a Hay first, in which we have an author in a hostage situation. Blink once if you can hear us, Sarah, twice if [Mark] Zuckerberg is an asshole.”

In testimony before a Senate judiciary subcommittee last year, Wynn-Williams alleged Meta worked “hand in glove” with China over censorship tools – something the company has denied.

The Republican senator Josh Hawley claimed at the hearing that Wynn-Williams had been threatened with a fine of $50,000 (£37,000) every time she mentioned Facebook in public. However, the BBC reported that, according to Meta, she faced paying those damages for each violation of the separation agreement that she signed when she left the company in 2017.

The Labour MP Louse Haigh claimed last year that Wynn-Williams was being “pushed to financial ruin” by Meta’s legal stance.

Meta declined to comment directly on Wynn-William’s Hay appearance. It has previously described Careless People as a “mix of out-of-date and previously reported claims about the company and false accusations about our executives”.