Why would a steam rice cooker suddenly make my dad sick?
The big question is: what changed? Some possibilities:


The rice cooker isn't being cleaned properly. Doesn't seem likely that the residue on an even poorly-cleaned metal pan would significantly contaminate the results.
The water or other cooking ingredients. Doesn't seem likely since the water is probably the same stuff you drink, and generally there aren't any other ingredients when using a rice cooker.
Your dad. Has he developed some sort of allergy or reaction to the rice?