If it were my decision, I would take the cat for treatment. There could be two possible outcomes:

1) that the owners WERE attending to the injury, and therfore your intervention wasn't really required (but a wise precaution)

2) that the poor animal is living in pain, and needs someone to take charge of the situation.

Other people have said it better than I ever could:

Edmund Burke:
The only thing necessary for the triumph of evil is for good men to do nothing.

George Bernard Shaw:
The worst sin toward our fellow creatures is not to hate them, but to be indifferent to them; that's the essence of inhumanity.