The anguished mother of a British man who vanished in Sardinia this summer has urged a woman from Jersey, who he was with in the days before he disappeared, to come forward and assist with the police investigation.

Michael Frison, 25, from Chard in Somerset, went missing in mysterious circumstances from a farm in Luras, a remote, barren area in the north-east of the Italian island on 13 July, the day he was due to return home from a holiday visiting his grandparents.

Police conducted an on-the-ground search, assisted by firefighters, sniffer dogs and hundreds of volunteers, but found no trace. The search operation was called off after just two weeks, although an investigation is ongoing.

Meanwhile, his mother Cristina Pittalis, originally from Sardinia, has travelled to the island several times in her unrelenting hunt for clues, saying there was “no reason” her son, a swimming instructor, would have just disappeared.

“Michael would never have willingly left his family – he would not have done that to us,” she told the Observer. “I will never give up looking until I have answers.”

Pittalis believes some of those answers could lie with a Jersey woman called Niomi, who she had never met but who joined Michael about a week into his holiday. Niomi needed somewhere to stay, so Michael asked his grandparents if she could stay with him in their home in Sassari for a few days. The pair then travelled to Luras to volunteer on a farm in exchange for food and accommodation.

“Michael contacted me to say Niomi was expecting her father to arrive in Sardinia,” Pittalis said. “He felt uncomfortable leaving her alone in a foreign country and went with her to this farm. The plan was to stay there until her dad arrived and then he would come home.”

Pittalis last spoke to Michael on 12 July. According to accounts given to police and Pittalis by Niomi and the farm owner, Michael returned from a walk the next morning in a confused state of mind, possibly due to heat-stroke. They put him in the shade and gave him water, only for him to go for a second walk and never come back.

The shorts, T-shirt and trainers he had been wearing were found about 200 metres away, meaning he would have walked off in his underwear. All his other clothes, documents, laptop and mobile phone were found in the tent where he had been sleeping.

Niomi left the island soon after and has since been uncontactable by Pittalis, despite several attempts. There are no accusations against her. “But it would be useful for her to tell me the story again, and the police too, to understand more about Michael’s state of mind,” said Pittalis. “There are so many things that just don’t add up.”

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She added: “It’s impossible to believe that Michael would have just gone for a walk, naked, in such impervious terrain – the land is full of scrubs and thorns; you cannot walk on it – and in temperatures of 40-45C. My motherly instinct tells me that he was taken, that others were involved.”

Michael’s disappearance came after the high-profile searches this summer of TV presenter Michael Mosley in Greece and Jay Slater in Tenerife. But there has been little coverage of his story in the press beyond Italy. The trip to Sardinia had been a 25th birthday gift from Pittalis, who also has a 10-year-old son.

“Michael is an amazing young man who never judges anyone and is always happy to help people,” she said.

“He adored and was very protective of his little brother, who is heartbroken. Michael and I are very close, and he always had the freedom to live his life, which is why I don’t accept people who say he just wanted to disappear.”

Pittalis said there were no signs that Michael was struggling with mental illness: “He was happy; he was sunny.” Police in Sassari did not respond to a request for comment.

Hundreds of volunteers accompanied Pittalis on a recent search on the island, and she will return on 6 December.

“I am constantly appealing to the public to keep awareness high, as if there is no body, it means that Michael is still alive,” she said. “It’s horrible not knowing. We’re living in limbo. It’s extremely difficult to sleep. But I will never, ever give up.”