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Hi there, this is Dr. Elaine.
There isn't anything in sweetened condensed milk that is toxic, but there is a whole lot of sugar. Lactose as well.
I would expect you might see some diarrhea because of the sugar concentration. Unless your dog is diabetic I wouldn't worry about this being dangerous.
Feed something bland when you feed again--wait until morning to feed her, but she can have water overnight. Bland for dogs is white rice mixed with cottage cheese.
This should help with any diarrhea that may develop.
If additional symptoms occur or the diarrhea becomes severe, lasts more than a day, or vomiting ensues, she'll need to be seen. I doubt that anything like that will occur.