My girlfriend goes to the chiropractor once a week for an adjustment. Dr. Uses a spring device. Is this effective treatment? Seems odd to me.

Unsure. Can you be more specific as to what is being adjusted. What kind of spring device and how is it applied and where?
No. The spring device is an assist to the sometimes dangerous and vigorous massage that chiropractors use. If your girlfriend has a recurrent problem, she should see her family doctor or an orthopedic physician to get it diagnosed and help to provide a lasting cure.
Chiro activator. This spring device is known as an activator and is a quick pressure delivering device that has both critics and advocates among the chiropractor community.