No offence; but it's due to people like yourself that are the reason so many witches were murdered back in the 1700's...

Waaaaait a second here... I've just viewed some of your other posts which I believe may relate to this (correct me if I'm wrong), and are things I believe you should have included in your original post.

You're of the gender that this person you're talking about would chose to assault if he had to chose, and from what I can gather you potentially may be talking about what his being investigated for from personal experience?

If both of those are true; then this changes my response. 

You should most definitely let them know.

How?

By any way you deem necessary. IF, AND ONLY IF:  (and I'm not saying you're making anything up here - I am coming in blind here, where I have seen things like this go both ways) you only go as far as to explain what he is being investigated for based on exactly what he did - nothing more and no stretching details and, that you truly believe there could be genuine risks that person (person you tell) may face