can be a difficult medical problem, and I definitely recommend that your fiancee talk with his primary care doctor
to help sort things out.
I would be very surprised if his diabetes were the cause of his insomnia, unless specifically he is having problems with neuropathic pain related to the diabetes or something similar that is keeping him up at night.
It is much more likely that he is suffering from insomnia for the same reasons that others, including nondiabetics, suffer from insomnia. Usually insomnia is related to stress or problems with sleep hygiene. There are numerous lifestyle changes that can be made to ensure relaxation in the evenings and promote sleep. Avoiding naps during the day, establishing a strict sleeping schedule, and the like can also all be helpful to improve sleep hygiene. Avoiding caffeine and alcohol is also important.
If these and similar lifestyle measures do not work, then it is time to talk to his primary care doctor about which medications for insomnia might work best. Luckily, these medications do not have an effect on blood glucose levels, so they will be safe to take with his insulin and should not mess up his diabetes control.