I can honestly say that I've never seen a problem like this before.
To me it looks like a lens flare, external light coming from the right. I had a 12-120 from the early 70s that would catch light and flare. Problem is, we don't know much about the conditions of the shot. Perhaps other examples of the problem might give a clue. Was all the footage shot by the water? We don't know that.
As for this screen shot, questions: What focal length was used? What stop was it shot at? Was the effect the same while zooming? Was the effect more visible at the long end of the lens? Was the effect more visible at the short end of the lens? Did the effect change shape or size at different focal lengths? Sunshade used? Matte Box used? Filter or filters? Polarizer?
What camera was it used on? Now that he knows what the effect looks like, can he see it when looking through the lens on a reflex camera?
It could be that the coating on the rear elements are bad on his 15-300. They are closer to the film and the defect would be sharper (focus wise) than any defect internally. If this 15-300 was originally designed for video cameras in the 70s or 80s, is the rear optical group bad?
He said that footage shot with Zeiss primes was ok. No comment.
I would shoot a test with the lens under varying light conditions, pointing a flashlight into the lens to create flares, to see if the effect happens again. Shoot it with the lens wide open, then in higher light levels with the lens stopped down.
If it's an internal lens problem with this 15-300, and all footage shot with the lens has the same, or has a similar look as the example pic, then he should have the seller check it out, or have a lens tech familiar with Angie lenses look at it. Or, he could return it and get his money back. As you know, repairs can end up costing as much, or more than a used lens.
Visual Products has one listed.....
Or, he could just use this lens when shooting B&W film.