The funny thing about Japanese law, their copyright laws are so strict that you can barely get away with making what would be a fair use project in other countries, but when it comes to types of fiction, as long as it doesn't involve real people, even this stuff is technically legal (basically an 'it's just fiction' mentality vs their overzealous 'protect the owner' mentality about copyright. Some other countries don't even allow written depiction, literally just words like my comment here, of a character that is or looks underage, but Japanese laws on fiction basically allow anything that doesn't involve real minors in sexual situations, real meaning a real person who is underage, as long as release of the work is restricted so that real people can only see it if they are at or above the age of majority. On the other hand, you have Japanese copyright law, which has no fair use clause, but enforcement is focused mostly on companies to keep them from stealing competitor property or secrets. As an individual, a Japanese person could probably get away with using copyrighted materials in a fair use sort of way, though it technically would not be legal, but a company would be hit hard and fast for the same). Don't get me wrong, they TRIED to ban the stuff, multiple times, but it has failed. They have passed laws against actual real life stuff, the biggest being the possession ban of 2014, but have yet to ban anything not based on a real person.
For the reasons above, I have to agree. He would not only be dealing with the insanity that is Japanese copyright law, but would also be dealing with other countries and their insane content laws, especially countries in Western Asia and in Europe, some of which fall into the 'even words describing it are banned' category. He probably wouldn't have trouble in the US, he would probably have issues with who to go to for distributing it since a lot of platforms do ban it even if it isn't illegal (the content itself is protected due to falling under first amendment protections afforded by the supreme court decision in Ashford v Free Speech Coalition, but is still subject to US obscenity laws, which are state by state), but once he has a platform, he shouldn't have issues outside those states that cover the content in their obscenity laws.
I would have to say this would not be worth the hassle for him trying to put out an unofficial translation, there's just too much headache in Japan alone.
