Congratulations, transliterating to romaji is already more effort than a mangaka should have to spend on something that works fine as-is once explained to please a single complaining foreigner who he doesn't know exists. The point of the manga is that what the characters are saying is already vague and relies on context in-universe to figure out an entire new language, readers shouldn't have to go through the extra hurdle of reading in their least common alphabet. If anything, you could've said " the author should have written everything in katakana," which would've worked just as well since each character corresponds to one syllable, but this just shows how you don't actually give a shit about how easy it is for the Japanese audience to parse and are just trying to justify your own stupid fix to your anecdotal problem that only you have.