Reports I have read say fragging was MUCH more common in rear areas (2 to 1 ratio) and much more in the Army than the Marine Corps. This cite says in 1970 US Army rate was 0.91 attacks per 1000 soldiers, while the 1st Marine Division had 0.2 attacks per 1000 Marines. The Marines took steps to stop them, and the reported rate dropped in 1971, while the Army rate doubled to 1.8 per 1000 soldiers.
While it not impossible at all the story has a ring of truth, it would be statistically much more likely if he were an Army veteran, or a Marine in the rear (support forces, non trigger pullers).