Water changes depend on the amount of fish, the size of the fish and the size of the tank.
I have heard of low stocked large tanks going months without water changes so I know it can be done.
My boarded tanks went about three- four months without a water change. Some of the fish died ( maybe one -two?) in the larger tank during this time and when I got back I did a rather large water change. I'm not sure how many because they arn't mine and I don't keep track of them. All I know is I found fishy bones in the gravel when I was cleaning. In the second smaller tank, all of the fish lived.
I knew large water changes were bad when the water quality deteriates so to avoid shocking the fish, I did a fairly large water change and then refilled the water back in over a few days. None of the fish died.
The other smaller tank simply got more frequent water changes for a while because there was no problem issue such as dead fish in there.
My usual routine for my boarded fish is one 10 percent water change a month, unless I'm trying to breed them, then more. Sometimes when I'm over there I'll also top off levels from evaporation.
Without going all into my personal life here I am trying a new med. and if it works I would like to work overseas for a while. Low care setups are a must when you plan to do things like this. I've always wanted to do this and have been held back by money/ getting an overseas job and because I get sick every two weeks ( which kinda interferes with holding down a job).
So I'm fixing up two magnum canister filters ( got the parts on my desk right now..$$$ for me) and when I find the replacement parts later on, two more power filters will be fixed up, depending on $ and how much space there is to put filters on.
Also I've been meaning to work up to hitting the trail again for some backpacking. Since the longest I've been out on the trail is about a week, this won't be much of a issue with my current water change routine being a month.
I'm just not serious enough to subsist on weird hiking foods in the wilderness longer then a month hehee.
These are the fish I have in my boarded tanks.
150 gal: Around six- eight of the medium cichlids left ( I don't keep track of them) and three feather fins. Yes the featherfins are what I get my namesake from. Fun dancing catfish.
55 gal: Four clown plecos. During a prolonged absense such as going overseas I will move the killifish planned for the appartment tank in to this tank. Other times the killifish will live with me in their landlord regulated small tank.
Those CPs are hardy little catfish: Without going into detail, this involves moving and boarding situation.
They will survive being moved an hour away in an ice cream container. ( I lost other fish during this move!)
They will survive moving complications ( only one had to go through this because si was missing during the move and had to be moved around later)
They will survive living along side a dead four inch rotting tankmate estimated to have been dead for two weeks.
They will survive living without filteration for two weeks provided that water changes are increased.
They will survive a few days of 55- 60 degree temps . This is the room temp of the cellor they are being kept in and it had to be done because the heater was going bad and making the water "tingle".
I cannot say the same for the synodontis cats. While I still have my original trio, they have had health issues ( ick, heater burn, injuries from me catching them and injuries to their barbals/ syno barbals are sensitive) in the past and get thin fairly easily in dirty water ( I assume they stop eating when the water quality gets bad). Seems like they can take it, but they'd probably starve if the bad situation continued too long. That they are slow feeders and and the cichlids eat much faster is another factor in this case.
Your friend might want to work on their setup to make it lower maintaince if that is what is desired. Plus some good hardy fish ( like clown plecos apparently) can't hurt.