It's a whole lot more dangerous to the person that cuts the string than to the bow, but there is still a lot of energy being suddenly released. There have been a couple of reports of people being killed doing something like that. Extremely rare, but possible.
Why did he cut the string? Or did it break at just that time? If the string was already damaged enough to break, he should have put it in a press immediately. Or maybe he didn't have time to do that?
The first thing to look at with a fraying D-loop is your release. There is probably a burr on it that is damaging the loop. Often easy to fix with a file and / or sand paper. A D-loop will last almost indefinitely if it's not damaged. Immediately stop shooting and find the reason for ANY fraying. It will almost always be your release.
If this guy cut the string on purpose, you might want to have the bow checked out and repaired by a more competent tech.
Allen