To me it looks like it had mites. The mites will get under the scales and feed off the blood of the snake. Once they're killed there can still be red marks from the blood of the area that the mite was feeding on. Its not noticable on other snakes because of their color but on a white snake it shows like that. Even if it sheds there can still be red marks because of this until they fully heal. I'd be willing to bet your friend had mites while having him.