What do you do when insurance thinks it should be workman's comp, but it's NOT?

My fiance works as a union laborer, so he has a tough time corresponding with his insurance company during business hours. He doesn't really know how to use computers that well, so I do a lot of correspondence for him. Well about a year ago his doctor said he should go to physical therapy (there is a note in his chart showing it was medically necessary) for his bad back. The insurance company insisted they needed a letter from him stating what the injury was. Well there was no injury, just wear and tear. So I wrote that his bad back was exacerbated by his work. I did NOT write that he suffered an injury while at work. So now the insurance company says it's a workmans' comp issue and they are refusing to pay the $2800 for his PT. What do we do? Is he now forced to make a workman's comp. claim or can he prove to the insurance company that he is NOT seeking workman's comp? Is he now stuck with the bill because I worded the letter improperly?