I had C-6 surgery on March 31st of this year. Prior to the surgery, I had a constant knifing pain in the left shoulder blade area and on-and-off radiating pain down my left arm (felt like when your arm "falls asleep") and numbness in my forefinger and middlefinger. This all started in late January. I tried all sorts of pain management including steroids, which did not help. Finally, I relented to the surgery. They went in thru the front of my neck, since the back of the neck has so many tendons and muscles. Upon waking from the surgery, I had immediate relief from the pain in my back/neck/arm. The only post-op pain I had was a bad sore throat for about four or five days due to the intubation tube. They gave me Percocet, which I did not need. The most difficult part was not being able to lie down flat for the first couple of weeks while sleeping, but you adjust. The doctor (who was great) put a piece of bone from the bone bank where the disc had been and fused the vertabrae. A titanium plate was also screwed in place for stabilization and quicker recovery time. I was in a hard aspen collar for two weeks and then a soft collar for four weeks after that. I was driving and back to work within 3 1/2 weeks. The doctor told me no contact-type sports for six weeks but by 12 weeks he said that I could whatever I wanted to do. They gave me the option of physical therapy but I declined and have felt great since the procedure.
Whoever mentioned lack of mobility or loss in range of motion is mistaken. I was concerned about this and asked the surgeon prior to consenting. He said that the range of motion in your neck is mainly in the C-1/C-2 region (near the top, towards the skull). Surgery in the C-5/C-6 range should cause little to no loss and this was the case for me. Good luck to your sister-in-law.
BTW, I only have a small scar at the base of my neck that is about 1 1/2 inches long. I know several people who have had this surgery and they said that after a couple of years they can barely find their scar anymore.