Well, if that were entirely true, we wouldn’t be having this conversation. 
https://www.quora.com/Why-are-underscores-not-allowed-in-DNS-host-names
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.)