Advice on alerting employers to an employee's past illegal activity?

I know someone from a troubled background who has an extensive (and recent) history of violently abusive behaviour towards women. This behaviour was investigated by police, but the investigation occurred months after the fact and they concluded nothing could be done about it.

As of a week ago, the man in question is employed as a security guard. In addition, he works parttime as a teacher at an ESL school for young children. His longterm career goal is to teach.

The idea of his having such a degree of influence and authority makes me feel sick. My question is:

Is there any way I can alert his current/future employers to his (off-the-record) record of violence? Any advice would be very much appreciated.

The investigation was by one officer briefly looking into allegations that he was stalking an ex-girlfriend he had previously abused. The abuse itself was never directly investigated.

Is there any way to do THAT after the fact, or is it true that if no one came forward at the time of abuse, nothing can be done about it?

I don't think you should be involved. If there was an investigation, with merit, then it will be a part of his record.

Any school board or security agency would discover this in a background investigation.

Otherwise he has no record.

before you go and ruin someones life make absolutely sure the accusations you are making are true and that this behavior is in direct conflict with his jobs. Make sure you have the facts and the right info. and teaching and security guards do thorough background checks before hiring.
and there are whistle blowing numbers or hotline number that you can call and make a complaint or give information about.

"This behaviour was investigated by police, but the investigation occurred months after the fact and they concluded nothing could be done about it."

Then legally, nothing happened and if you go around telling people otherwise you open yourself to being sue by that person.

"Is there any way I can alert his current/future employers to his (off-the-record) record of violence? Any advice would be very much appreciated." - Sure, you could call them and tell them. And then he could sue you and win. No charges were filed.