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elefantine97 Yes, and no. The legal age of marriage for a girl in Japan is the bit that's 16, and that can negate it being "fornication" for the purposes of the child protection law that doesn't allow fornication with anyone under 18 nowadays.
But I think her parents might be against her marrying her friend's dad, so they likely wouldn't sign off on it and now the 18 thing is right back in place, since she can't legally get married without parental consent until she's 20.
Some of the prefectures might still have local laws putting the age of consent lower than 18 I guess, but it'd be the same situation as legalizing marijuana that's going on in the states right now, I think? The child protection law should be akin to federal law, while the prefectures would be state law level. In theory, you could still get busted for breaking the law in those prefectures because the child protection law would take precedence.