I think that what your friend is doing works on very low light tanks. Very low light = no ferts = less need to change water due to fert build up. He probably just barely feeds the fish, less in = less out.
Also, it is very important to know what is in your tap water, if it is loaded with phosphates and nitrates, then water changes could be a bad thing, or at least something you'd want to limit. many places in Europe and certain farm area here have so much stuff in the water, they practically dose tap water to add the nitrates and phosphates they need.
As far as the UGF, that would insure that whatever is in the gravel is in the water and whatever is in the water is in the gravel. Now most of us don't want to have everything in the gravel in the water since we have added fertilizers under there that might create green water if circulated through the tank. But, if you don't add fertilizers to the gravel and don't change water it could work to circulate this to the roots. Otherwise you'd have nitrates building up, phosphates too, unless you added a lot more plants and filtration. Adding circulation through the gravel in effect increases the size of the gravel bed.
Then, the danger involved in changing or not changing water depends on whether your tank has stable parameters, for if you had a low KH to begin with and never changed water, the KH would drop lower and lead to an unstable pH, which can kill fish.
Looking at this from his point of view, as he watches all the work you are doing, directly due to the high light levels, he would tend to think you are wrong. In about 6 months, your tank will be lovely and lush, but you will still be doing all this work, cutting plants instead of scraping algae. At that point, he may still think you wrong, just based on the work involved.
On the danger of water changes, he may be correct if one day you do a large water change only to find out later that the water company has just treated the lines with some noxious chemical that your dechlorinator does not touch, because of some line repairs that you had no knowledge of and even if you did you never would have been able to test for that chemical.
He, on the other hand, risks killing his fish with the infrequent water change, if the tap water hs become so different from his tank water --, perhaps from evaporation and topping off -- leading to a concentration of minerals and risinig GH. Then a sudden lowering of the GH from water change can do damage to fish.
In the long run, it is better to keep the tank near to tap levels, for when things go wrong large water chnges should be your friend. And we do that by doing regular water changes.
Light is the driver -- the light you run will dictate what is the better practice. More lights means more ferts means more water changes to keep from overdoing any ferts.