The heart beat in the neck isn't common, but my gelding has it too, it's very visible. As long as it doesn't travel up the neck, above the heart it's an 'okay' thing. My horse is different in that his temperature is extremely low, like 94. But he has a large edema on his stomach behind the girth line. Pathology wise his white and red blood cell counts were very low. We're watching him very closely but think it's lymphoma, which can end in colic. From what I gather lymphoma can effect the heart.
Not sure if it's the same thing with your friend's horse, but could be a possibility??? Did the vet run bloods?