Too bad that's not how theobromine poisioning works. Technically it would be just as poisionous for humans as it is for canines, but our increased body mass and larger livers make it nearly impossible to poision ourselves faster than we metabolize it unless one were already in liver failure. Even if Momiji did process it as slowly as a midsized canine, it would take more than a few first-time bites of dark chocolate for symptoms of poisoning to manifest. She would need to eat a significant (several bars) amount, or eat moderate amounts at regular intervals (before it could be metabolized) for her to start feeling sick.
TL;DR unless Momiji has a tiny dog liver, she is virtually unaffected by the theobromine in chocolate.