(not a vet, don't try it in real life when you encounter her) Not enough chocolate to poison here, if you want my opinion.
Considering Chen should weight at least 40 kg (~90 lb / btw, I really don't know how much should a cute catgirl weight be, so don't throw rocks at me for that), and her metabolism should be somewhere between a cat's and a human's one, according to Wikipedia (http://en.wikipedia.org/wiki/Theobromine_poisoning), I think she'll need at least half a kilogram (~17 oz) of black chocolate to start experiencing intestinal distress.
TL;DR : It should be fine, need moar « cute Chen eating Pocky » pictures.
I find it hilarious how whenever Momiji or Chen are shown eating chocolate, there will always be someone mentioning poison while someone mentions they will be fine.