If Bob's speaking in the present tense, then "can't" (he's still laughing at the joke now), as "couldn't" would imply past tense (he was laughing at the joke, but not anymore). So they would both make sense, but have different meanings.

What a joke I can't stop laughing > You still cannot stop laughing ( even now) What a joke I could not stop laughing. > When you heard the joke , you laugh . Now , looking back , you say , "I couldn't stop laughing (then) " . But . you don't now.

From John's remark, the time frame is placed in the past. So it's more likely that Bob also came across it in the past, and thus "couldn't" is called for. But it's possible that he only just saw the joke, in which case the present tense "can't" is right. (Except that he wouldn't be able to say anything, because he's laughing so hard ...) By the way, the punctuation is incorrect for Bob. The remark needs a stop of some sort after "joke".