Looking at the facts we know I can see how it'd be Ben.....
- Jacks says it's neither friend nor family, Ben fits that mold.
- Kate didn't want to go
- Nobody showed up for the funeral which makes sense as nobody would know Ben made it off
- Timeline wise they could establish that Ben got killed after the stuff with Sayid was over
The one thing that goes against this is the name in the paper started with a J and while it's possible that Jack knew one of Ben's aliases I doubt it. Also, for whatever reason I can't see Ben being bured in L.A. Do we know, based on the paper clipping where the person in the coffin died?