Laleh Shahravesh, 55, faces two years in jail and a £50,000 fine over posts allegedly written three years ago after she discovered that her former partner, Pedro, had remarried, the Detained In Dubai group said.
In one post, she wrote: “I hope you go under the ground you idiot. Damn you. You left me for this horse.”
Ms Shahravesh, from Richmond, south-west London, was arrested with her daughter Paris, 14, at Dubai Airport when the pair visited on March 10 for Pedro’s funeral, one week after his death from a heart attack, aged 51.
They were held for 12 hours before Ms Shahravesh’s passport was seized, Detained In Dubai said.
She had been married to Pedro for 18 years and they lived together in Dubai, where he worked for HSBC, for eight months before she returned to Britain with their daughter.
A few months later, in 2016, she unexpectedly received divorce papers and saw from photos on Facebook that Pedro, who is Portuguese, had remarried.
In another comment, which she wrote in Britain, she said: “You married a horse you idiot.”
Detained In Dubai said her arrest, under strict cybercrime laws which also include a ban on sharing charity pages online, was “simply unreasonable”.
The posts, written in Farsi, were reported by Pedro’s new wife, Samah Al Hammadi, 42, from Tunisia, the group said.
Ms Shahravesh said: “I had no idea he was getting married again, and so soon after our own marriage broke down.
“I reacted badly. I lashed out and wrote two unpleasant comments about his new wife on his Facebook page.”
Paris has been allowed to return to Britain but Ms Shahravesh must remain in the country and faces further court proceedings on Thursday.
Ms Shahravesh added: “I am terrified. I can’t sleep or eat. I have gone down two dress sizes because of the stress.
“And my daughter cries herself to sleep every night.
“We are so close, especially since her father left us and we only have each other. It breaks my heart to be kept apart from her.”
She said she has lost her job at a homeless shelter, could lose the flat she shares with her daughter and has borrowed £5,000 from her family.
“My life is in ruins, and that is even before the huge fines and jail I am facing here,” she said.
Radha Stirling, chief executive of Detained In Dubai who represents Ms Shahravesh, said the country’s cybercrime laws render “almost every visitor to the country a criminal”.
“I have spoken with Laleh, her mother, sisters and daughter Paris,” she said.
“Their experience is heartbreaking. Not only has Paris lost her father, but in going to visit him to say her final goodbye, she wound up in a frightening Middle Eastern police station, and is now without her mother.”
A Foreign Office spokesman said its staff are supporting a British woman and her family following her detention in the United Arab Emirates.
The spokesman added: “We are in contact with the UAE authorities regarding her case.”