He clearly chose the best starter.
But I wouldn't say sun/moon were easy. I mean with the way I played it was more difficult to the point where I got rekt by an alolan raichu due to being under levelled. But I will say that a nuzlocke of sun and moon is something that will be hard especially US/UM. There are multiple totem pokemon that come to mind that will in fact kill you unless you overlevel. The grass one stands out, mimikyu could do it. And in US/UM the dragon psuedo legend... who I was multiple levels higher than surprised me when it took a moon blast due to its item then proceeded to destroy my team actually forcing me to use items. ORAS is the worst remake of a pokemon game hands down and it is too easy. But you wont have the problem with sun and moon, i've seen people do them... the majority fail, pretty early on.
Water type is the best starter in some Pokemon games. But Sun/Moon aren't among them