My dog also ate a chocolate candy bar and was told by my vet that while he would possibly throw up and have loose stools it would not kill him just make him possibly sick. Don't panic just call your vet and see what they say. My dog was fine and has no issues from it.
Don't call your vet, they'll say to bring it in just so they get money out of it. Chocolate is NOT poisonous to dogs. I've raised dogs my whole life and a few times they have snuck in the kitchen and found candy they shouldn't have eaten but not one of them got sick
Just wait to see if she starts acting funny. A little bit of chocolate won't hurt the dog. If he/she ate like 4 bars then yes you should but just one shouldn't do much but get her an upset stomach. Keep an eye on the dog.