the muscles in your stomach are like if you hold your hands, fingers entwined. tight ab muscles...like a tight grip. as the stomach muscles weaken and the belly protrudes, it's like pulling the hands a part. the bands of muscles can't hold together.
some people have a genetic weak area in the abdominal muscles. from my experience in the gyms, it's seen more on the "beer belly" types. it does not mean the persaon drank beer to get that belly. many factors can contribute to that type of "gut".
i really am surprised his dr has not rx'ed exercise to strengthen his ab muscles to keep any other hernias from developing.