If everyone who made a mistake caused by factors they were not aware of just stopped doing things, then no one would ever do anything. Even you, making this kind of hypocritical. She's still centuries older than these boys, and thus more experienced. Doesn't mean she's infallible, especially when dealing with another ancient demon/spirit, but it does mean she should be many times more reliable than either of them.
They're, after all, like infants compared to her, in terms of worldly experience and knowledge. Thus her wisdom is higher.
@Adabido against the enemy of the whole series since the first few chapters when MC comes back to his home town no ? fucking hell can't even follow a story