I mean I think it’s the adults responsibility to make due diligent efforts to check, but if the minor continues to lie, is it really on the adult anymore? Unless there were glaring red flags that a reasonable person could suspect, I don’t believe the adult can be held responsible.

I have no idea how the law handles this, but from my understanding, “mens rea” (guilty mind) is required to prove criminal wrongdoing. There would be no “mens rea” proof here since the adult had to have willful intent to commit a crime.