Assuming the chap is telling the truth - and you're right, he might be lying - I'm basing my reaction to this partially on situations I've encountered at work.
A colleague in a Scottish high school was punched by a teenage boy. He put out his hand and blocked the second punch.
The police came to the school, interviewed staff and said that there was no case to answer. However, the boy's parents disagreed and complained to the Director of Education for the Local Authority.
The result was that the teacher was suspended, pending a full investigation. There was a tribunal where it was acknowledged that the teacher had been hit, but he was told that he should not have stretched out his hand; he should have 'run away'.
The teacher returned to work, but was left so vulnerable as a result of what had happened that he quit at the end of he academic year.
I admit to being biased. In a similar incident, I tried to get away from a violent teenager and that didn't work.
The teenager who attacked my colleague towered above him and was a violent thug. The punch happened during a fire drill and the teacher wasn't even interacting with the teenager. (He was arrested for another crime a few months later, as soon as he turned 16 and could be referred to court rather than the Children's Panel in Scotland.)
If the prison service regulations and norms are anything like those that apply to teachers, I suspect that the potential date was held to higher standard than a member of the public and that that is why it went to court.
However, you're absolutely right - he might be lying and since that seems to be the OP's fear, she should absolutely discount him as a potential date.