Well, if that were entirely true, we wouldn’t be having this conversation.
The best solution, in my arrogant computer geek opinion, would be to point the guy to the above web sites and suggest to the guy that he’s doing it wrong and that if he wants to link to his site he should create a CNAME using a - instead of a _. (You’d probably want to find a nicer way to say that.)
And what about IDNs? The regular expression validating the website url is so strict that I can’t fit in my perfectly valid URL https://ondřej.caletka.cz
Even using Punycode form of https://ondřej.caletka.cz is not accepted, even though it is perfectly valid here in this very forum post.