personal responsibility is gone
She wanted an assurance that it wouldn't happen again. I re-explained about how filters aren't perfect, and told her a determined kid can always find something that the filter doesn't catch. I ended by saying that it's the parents responsibility to supervise their child, and if they don't want the kid using the computer when they're not here, they should take away the library card.
Her response? "I think it is the library's responsibility!" Uh, no. I am not the parent. It is not my call whether any specific item is or is not appropriate for your child. That is a PARENT's responsibility. You don't like what you kid's up to, then supervise them more. Don't tell the school or the library it's their job. It's not, and there are a lot of other parents who would take serious umbrage if they were to try.
Oh, and she also thought that the filter should be on ALL the computers - I explained that adults have the right to look at whatever they want, but she didn't much like that statement either.
ETA: Look, a comic on just this topic!