My grandparents haven't cleaned the condenser coils at the back of their fridge for 6 years, and worry that it's grubby. Their fridge whirs louder and louder, and they feel heat when they place their hand near the back. They're too frail to pull it out.
It pains me to provide a "BUY SOMETHING" answer on Lifehacks — for a problem (in this case) that does not need a lifehack so much as physical effort. But you asked about something that someone suspects has a problem but can't or won't be verified and will be treated "in-situ." There are many other cases where circumstances prevent an obvious remedy. In that spirit…
There are many special attachments for vacuum cleaners for pulling grunge (dust, cobwebs, lint and hair) from hard-to-reach places. These things are made to reach into all those limited access places - like heating vents, grates, dryer vents and lint traps, between radiators and under and around refrigerators and ovens. Buy or rent one or more gadgets that appeals to them. If your elderly relatives will have something to use, maybe they'll use it more often than once every six years.
While an air compressor or pressure washer would probably do a better job of removing the dirt, A vacuum cleaner for a residential location would probably be a wiser choice to control spreading this contamination around.
The very real difficulty your elderly relatives face is not being able to see what they're doing. That's not a trivial issue for anyone at any age. In this case, just do it anyway and hope for the best. Maybe doing something badly and inefficiently is better than doing nothing. At the worst, they'll probably dilute the gunk by spreading it around. In addition, they'll have to be pretty flexible to lie on the floor and climb on things to reach inconvenient places with the cleaning equipment. Who knows, maybe the exercise will be good for them at their age?
Wish them luck for me.
It is important to know exactly the placement of the fridge, and of other things around it. As in... do you have any access to the back?
If there are "big" things stuck to it, you can use a stick, or a strong(er) metal wire with a hook at the end, to take them out.
If it is dust, use some brush for dusting:
Of course, compressed air will work, but all the dust will then flow happily in the room.
If the space around the fridge is very crowded, then it is a pain in the behind. Of the fridge :)
One idea would be to create a string loop, larger than the fridge. Imagine a belt for the pants. But for the fridge. This string loop will go around the fridge, and it will be able to move freely, left-right and up-down.
Attach a fluffy something to the string in one place, "spherical" or "longish" (it must embrace the string entirely, otherwise it will not press against the dirt). The fluff will brush the rear of the fridge, dislodging the dirt.
For better results, a vacuum-cleaner's hose should follow the fluff, to add the dust. Otherwise, the dust will go back on the fridge, or in the entire kitchen.
NOTE: ask your grandparents to take a picture of the back of the fridge, using the smartphone's camera, with the flash on. If they are lucky, no cleaning will be needed. Alternatively, use a mirror and a flashlight (a bit more difficult, but doable).