If there is bruising, inflammation, disk disease or injury to
the spine the steroids may help, unless
the spine is severed or partially severed.
The rigidity to the neck that you describe is not a good sign.
Valium is used as a muscle relaxer, and works well in conjunction with steroids, which are anti-inflammatories.
In your GF's dog's case only time will tell. If he has paralysis as a result of this injury surgery may be required.