They could have prevented it to a certain extent. This case is such a clear-cut example of someone who should never have been allowed to be anywhere near women, but it could have been a case that's not so clear-cut. It could have been someone who's not particularly violent towards women.
It's important to make a distinction. They made a mistake in allowing this specific man to be around sex workers. They also made a mistake in assuming all sex work is wrong. Their objections to him seeing sex workers and the objections that still seem to be talked about in terms of why he shouldn't see sex workers is that sex work itself is wrong and no delinquent should ever see a sex worker. That's also a big mistake that was made.