It may not make you immediately a better pilot, but the bar for a cpl is a lot higher than for a ppl. Both in terms of a harder test, a requirement to have a lot more hours as pilot in command, to have an instrument rating etc.
Couple that with the fact that the theory qualifications are also a lot more in depth. You need to do cpl theory at the least, although most will do the ATPL(airline theory) exams instead. Even if it hadn't improved his flying to a huge extent, maybe the extra qualifications would have helped him to come to better decisions about when to fly.