Are you an SEO consultant or another type of consulant?
You really are not providing enough information to get back good feedback.
-Have the backlinks changed? -Did they do any link development? -Do they sell/buy links? -Have you checked to see if the traffic loss is due only to a drop in Google referrals? -Have you scanned for malware or signs of hacking? -Have you checked to see if another site stole content? -Are there any 302 or 301 issues? -Have you checked for messages in WMT?
I could go on and probably list over 100 possible issues. If you want a better response please do a little more research and then ask for insights. It sounds like you have many issues if you have been consistently losing traffic since the 2010 redesign. Good luck.