Miss Myles says there is "no way" Mr Kenzie would have ridden a motorbike in Thailand if he had known he was not covered by insurance.
The accident took place on the first night of their working holiday. His boss Shaun Smith also went to Thailand but flew back early to Australia soon after the accident.
So far, Mr Kenzie has incurred $16,000 worth of medical care.
"All the hospital is interested in is the money they are owed. They have been intercepting my calls when I try to reach Sean and demanding to be paid," says mum Nadine Bennett.
"Even if we could afford to pay for a medical evacuation team to get him home, he still couldn't leave Thailand because his condition is too serious right now."
The insurance company could not be reached for comment.
Miss Myles said Mr Kenzie was not aware that to be covered for riding a motorbike he needed to pay a higher premium.
Mr Kenzie, a roofer for Heritage Roofing and Plumbing, lives in Perth after relocating from Tauranga.
His older brother, Kane Kenzie, approached the Ministry of Foreign Affairs for help but was told they could offer no financial assistance.
Friends and family have donated more than $2000 to help pay for Mr Kenzie's treatment bills.
"We just want to get him home. It's been a pretty horrific ordeal so far," says Miss Myles.