I don't think I've ever known of anyone who has injured themselves by slipping on a banana peel. However, I've seen that in cartoons many, many times.
I even found one picture that makes fun of the fact that no one really slips on banana peels anymore:
It turns out, though, that a hundred years ago or so, banana peels were a legitimate hazard. As one blogger notes:
Banana peels were in fact dangerous, especially in the large cities of the East Coast at the turn of the twentieth century. They were so plentiful that in some cities, peels became a hazard. Yes, seriously. People fell and were injured.
The blogger goes on to explain how this became a recurring gag in theater and silent movies, and eventually the slip "made its way into American culture."
I'm not sure if it wouldn't be better characterized as a figurative language, as opposed to an idiom, but that's splitting hairs. Chances are very unlikely that our programmer actually sustained a career-changing injury by slipping on a bona fide banana peel.
I'd guess the person probably was on a downward spiral – perhaps because of a bad drug habit, based on the snippet you've provided – but managed to change their ways and become gainfully employed.