If he just wants to remove it for it to be gone (ie not actually transplant it or keep the donor alive, it's pretty simple. Requires a sharp knife and decent knowledge of anatomy, so yeah a vet could (that's what I am) or someone who did some research on anatomy could do that. But I suspect that if a doctor who performed real transplants looked at how it was removed they would be able to tell that it was not removed in a way that left the organ able to be used because 1) certain blood vessels and attachments need to be cut at specific points and 2) the donor has to be alive while organs are being harvested which generally means a need for anesthesia.
In theory, I guess someone could just remove the entire liver and vessels as quickly as possible and possibly end up with a transplantable organ (I know that your killer isn't transplanting them, but it wouldn't be an effective distraction if it's easy to disprove)
On to the rest, I don't know exactly how it would be done by a human doctor, but when I've needed to get to the liver, I make and incision through the skin, fat and muscles just below the ribs, on the front of the stomach. Then reach up under the ribs to touch the lobes of the liver. Just have to push small intestines and stomach out of the way.
To actually remove the liver, all the attachments and blood vessels are on the far side from what you see from an incision in the stomach (ie toward the persons head and back). The bulk of the cutting would be done be feel. (I've never removed a liver in a live animal, only during autopsy. But the anatomy between a human and dog is nearly identical in this part of the body).
To just cut it out and not attempt to stop the bleeding would only take a couple minutes if the person had a good grasp on the anatomy. And it would be horribly bloody on a live or very recently dead person (throw a gallon of milk on the floor and imagine it was blood)
Sorry for the wordiness. This was probably way more than you needed or wanted
