Since his rig operated without a problem for four years, I think it is reasonable to assume something changed, so the first step may be determining what changed and finding a way to stop that from happening in the future.
100% agreed. It’s possible that he just got unlucky — rodents can chew on wiring and even help bridge the gap, things can shift, moisture could have gotten in the wrong place. One less probable cause that I’d look into just in the hope of good news is that the short took place within something plugged in.
It’s a rotten problem to have. Anything relating to wire tracing when it’s not easily accessible is no fun.