According to the symptoms you’ve described (“cannot move the wrist or hand up or down”, “cannot straighten the fingers out “), it seems to me that your fiancé is experiencing a disorder of the peripheral nerves of the upper extremity. Which nerves are damaged and where the nerve lesion is located can be identified with a neurological examination and electromyography (EMG). An X-ray image of the cervical spine is also requested to exclude spondylosis as a reason for the nerve damage. Together you can consult a neurologist for the examinations, diagnosis, and follow up.