Criminal? I'm not sure, but I doubt it. A state might have a ridiculous "anti-terrorism" law which could cover this, but I don't really think it'd hold up in most cases. It'd be more likely if he was trying to freak a crowd out with a story of some deadly contagion than playing a prank on an ex-girlfriend. Now, if she freaked out so much she suffered some sort of harm, he could plausibly be sued for intentional infliction of emotional distress.
Anyway, on topic: a friend of mine once called up his ex-girlfriend on April Fools to tell her he was just diagnosed with AIDS. She hung up after screaming at him before he could say "April Fools!"