rdywenur, the dog is not hers, nor even a member of her household. By taking the animal out of the house without the owner's consent, she could be held criminally liable, no matter what her reasons for taking the animal, or where she takes it. If she were a cop, animal control officer, etc., it would be different. But pets are property under the law. If she were to swoop in and take the dog without the owner's knowledge and consent, she would actually be a risk for charges of theft.
Although the situation makes me screaming mad at the owner, I agree with chatty's method of giving repeated warning to the owner and offering to take the dog to the vet. As a bystander, she is doing all she can do.
(Not to mention the familial hell she'd be in if she stepped on toes in this situation!)
Do you see what I mean? That's why she's talking instead of acting. That's not HER dog.
Best wishes to the dog. I hope she recovers.