This question is actually dual, legal and ethical. Not being a legal scholar, I will deal primarily with the latter, though legal ramifications of intervening must enter into one's moral reasoning. Exposure to liability in virtue of helping someone has been limited in many jurisdictions by Good Samaritan laws, which protect those who offer assistance of various sorts to people in trouble. The protection provided by these laws is consonant with common sense and protects the Samaritan's right to provide first aid, CPR, and minimal medical assistance that many people are likely to have mastered. It certainly would not protect me if, without cardiac surgical training of any sort, I cracked a chest and administered heart massage.
What the Good Samaritan laws seem to say ethically, is that citizens should not be discouraged from helping others at a rudimentary level commensurate with their expertise by fear of liability. This sounds like a reasonable moral position. We don't want a society where fear of a lawsuit prevents its members from helping one another.
When the Good Samaritan is a person who is medically trained, albeit not a physician, I would think that morally, the public is entitled to expect more in the way of emergency assistance than it expects of an ordinary citizen. The more training and experience in managing that sort of emergency, the more the public is entitled to expect. Thus a military medical corpsman or a veterinarian, both trained to control bleeding, could be expected to accomplish hemostasis in an emergency. Veterinary training for the first 2 years of veterinary school is virtually identical to that of human medical training. At one point, the State of Wyoming considered creating a medical school and training the medical students for their first 2 years at the Colorado State University (CSU) College of Veterinary Medicine. In fact, a graduate of a veterinary school is likely to have more training in surgery than does a medical school graduate.
Given that tissue is tissue and bleeding is bleeding, I would think that a veterinarian facing a laceration emergency with a human could be morally expected — and permitted — to treat the emergency.
The same reasoning should be applicable on a case to case basis, and moral obligation to help, as it were, titrated on the basis of expertise. An academic veterinary heart surgeon with 40 years of experience who has worked in human hospitals during sabbatical leaves is probably more competent to help in the situation described in this case than a physician dermatologist or recent medical school graduate. If the law does not currently embody this insight, it should be modified to do so.
In the spirit of our discussion, Dean Lance Perryman of the CSU College of Veterinary Medicine has made a novel suggestion. He has raised the possibility that, in rural areas deficient in physicians, veterinarians could undergo some additional training analogous to physicians' assistants, in order to provide rudimentary medical care in such communities. In fact, this already occurs de facto in some rural areas where veterinarians treat their clients' cuts and even fractures.
In sum, as regards the case in question, the veterinarian may morally provide emergency help, provided that he or she does not exceed his or her training and comfort level.
I am grateful to Ralph Switzer for dialogue.