I was told by a non-Catholic that the reason for abstaining from meat on Fridays came about was the Catholic Churches attempt to boost the fishing industry back in the early ages. Could you please explain how it really came about and why it became a sin?

As with all “legendary” explanations, they usually have something factual to which they are linked. In this case, it seems to be the fact that several of the original disciples of Jesus (Peter included) were fishermen by trade. That, however, seems to be the only link that can be substantiated historically. There is no real evidence of subsequent popes having connections to this industry.

Unfortunately, most people do not understand the history of the practice of fasting (refraining from food and/or drink in general) and abstinence (refraining form a particular type of food), and seem to think of them as disciplines created by the Catholic Church for its own religious (in this case, irreligious) purposes.
Fasting and abstinence actually predate the Christian religion. These practices are part of nearly every religious tradition, and are well rooted in Jewish practice since at least the time of Moses. Jesus was familiar with the practice and explicitly approved of it, as recorded in the Gospel of Matthew: “When you fast, do not look gloomy like the hypocrites…” (Mt 6:1).

What is distinctive to Christianity, however, is the day of fasting. In the early Church fasting was observed as a regular penitential practice on Wednesdays and Fridays, as well as every day during Lent. In time abstinence from meat became particularly linked to Fridays as a way of commemorating Jesus’ passion and death (which occurred on a Friday). As the centuries passed, more and more days of fasting and abstinence found their way into the Church’s practice.
With the Second Vatican Council the Catholic Church chose to relax the disciplines significantly. Today, adult Catholics (18-59) are obligated to fast and abstain from meat on Ash Wednesday and Good Friday. Abstinence from meat is to be observed by Catholics over the age of 14 on Ash Wednesday, Good Friday, and every Friday during Lent.

Though some may argue that the Church’s official teaching never actually identified violation of the fasting and abstinence rules as a sin, the truth is that this was taught in many areas throughout the Church’s history. In response to this, the Church’s teaching since Vatican II has been that a spirit of penance and self-denial must underlie these practices, rather than simply adhering to the “letter of the law.”