OK...well, things are never simple, are they
Old...as in fuses, not circuit breakers. And, wiring throughout the house is likely of old type, poor insulation, perhaps of a small gauge. Thus problem could be anywhere , actually, that a high amperage device might be plugged in.
Applause to you for your concern and agonizing about what to do.
So, it now appears that the options are , in order of increasing expense
Educate the couple about the problem
insure fire alarms are installed in the house
assure that the immediate rats nest of cords is resolved and not pulling excessive amps
install a modern circuit breaker main service
replace wire/rewire entire house
I'm hoping that the couple are technically savvy and trainable so they can do some of the work themselves and live in the house in a way that avoids exposing the problem in a catastrophic way.
A "complete" solution would certainly be way expensive....even including your personal time and travel expense to pay them another visit.
But, at the root is the question of what money is for and what values and decisions a person makes in their life.....you can find these questions embedded in just about every TBN post/thread.
Thanks for sharing your concern with the TBN community...it puts a spotlight on the complexity and difficulty of being in "community" with others, be it at short or long distance, a close family member or distant, and how we interact to maintain pleasant and safe society. The tensions that make us human.
A personal story....my cousin visited the other day to see our fields of spring flowers...whole blankets of bluebonnets and indian paint brush...blues and reds spread across the pasture among the green of spring. Seeing our very friendly cat, she asked if he was micro-chipped ...nope, he roams 200 acres at will, avoids coyotes and hawks..so far..been with us 8yrs or so. She, being from the big city where apparently cats get lost, perhaps stolen, she believes every pet should be micro-chipped. She offered to pay to have ours chipped. I dunno. ..there are so many things I "should" do and have trouble getting around to...chipping the cat seems to be near the bottom of the list to me, but it's super important to her....