Yes, he might of been looking at the WTC and telling his friend about an 'I Love Lucy' segment he saw that morning on cable. . .
OR, he could have said. . .'just wait until they are gone and stupid America gets what it deserves. . .yes, that could be funny to the 'wrong' person. . .
While that moment is past, there is little to do about it; save return often to that restaurant, see if they show up again; get their license number and report them.
. . .OR. . .tell the FBI, if you have the exact date; they could perhaps, check restaurant records/receipts and bring up a name or two, that could be checked.
Alert is great. . .follow-up, as important.