Firstly I would just like to emphasise that whenever asking questions, please ensure that you provide all information in the question (don't just reference other sources, as we do not always have those sources on us, as well as it being proper forum etiquette).
So you appear to understand the concept of random censoring, and the idea behind this question is whether it is suitable to apply random censoring to the lives in the prison group (in particular those who left the study early). So as we are looking at mortality, we need to consider if there's any correlation between mortality and length of stay in prison (you could argue this either way - those that remain in prison longer have a higher risk of dying in prison fights or she to poor food or healthcare, or that those that have longer sentences may have worse lifestyles and be riskier, or that those that are being released are simply a random sample). Depending on your argument, you could this either justify the censoring as being appropriate or not. I do not have the memo to this question with me, but my personal opinion is that it is not appropriate, as the risks faced by those who will be leaving soon are significantly different to those who remain in prison, thus not making the censoring truly non-informative.