"I don't quite understand why using a single IV for the whole library is vulnerable to known-plaintext attacks."
That might be an acceptable response if you're trying to build a secure system and don't fully understand your tools yet, not many people do, but it should be followed with an eager question on how to improve. Instead he follows up with:
"I know it's better to use different IV and key for each file/block. But that would greatly increase complexity."
As if that's an excuse. And besides, solutions had been suggested and it's not that complex. Finally he just states that the security improvements are not scheduled.