Not that this will change your customer's mind, but for your own edification, read the latter part of
this post where I reference and quote the World Health Organization (WHO) summary of epidemiological studies on chlorine disinfection by-products (chlorine itself is not the issue; it's some of the chlorinated by-products that are mutagenic and in some cases carcinogenic).
The UV system really won't let them run at a lower chlorine level. The higher chlorine level is needed to kill algae. So if they truly want to operate at a lower chlorine level then they need a system that will kill algae in the bulk pool water. That means using an algaecide such as Polyquat 60, using a linear quat algaecide (that can foam), using 50 ppm Borates, using copper ions (that can stain plaster surfaces and turn blond hair greenish), or using phosphate removers. Neither ozone nor UV will do anything against pathogens or algae growing on pool surfaces and not getting circulated. With supplemental algae killing chemicals, they could operate their pool at a lower FC/CYA ratio. How low, we don't know since we don't have enough data for that, but I'd say it's probably at least half as low and possibly 1/3rd if the supplements are very good.
Also, as others have noted, since he is looking for convenience as well as low chlorine levels, he should get a saltwater chlorine generator for sure. Whatever he does beyond that is gravy. If he gets only a UV system, he still won't have automation of chlorine dosing.