Canon has only a few approved independent facilities which have access to parts, tools and software for calibrating lenses. You might get it repaired on the cheap, but then have to send it to Canon or one of the facilities to get it properly calibrated.
Your friend will probably not save any money on a repair of a lens that obviously will need re-adjustment.
One of the big independent repair stations is Midwest Camera, since you shoot Nikon, you might already use them.
http://www.midwestcamera.com/
Here is a ex Canon Repairman who is considered to be the best, his customers rave about him. He is good at repairing older lenses that Canon does not touch. I don't know about pricing.
This is where Canon goes when they need help:
T F Camera Repair
27 Brunswick Woods Drive
East Brunswick, NJ 08816
Phone: (732) 238-8806
