I spent nearly 2 decades in the Airline industry, the last miserable 6 years were with Delta. In the interest of brevity I'll just say this doesn't surprise me at all. But keep in mind it was just as likely the TSA as it was the inept staff at Delta. Someone probably opened the case for an inspection (probably looking for drugs), placed the guitar somewhere precariously and then the damage occurred. Then placed back in the case so they wouldn't be held personally responsible and then sent on its way.
Hope he gets every dime, but it might be a longshot and will probably hinge more on his flyer status than anything else. Taking it public like he did was the best approach, if it goes viral they're even more likely to buckle.