It's not the boss' job to investigate, buy it is his responsibility to report it to company security and file a Police report wether Mike did or not. It's a crime that occurred at the workplace. He was informed of it and looked the other way.
Its up to the company's security or police to investigate. If they find something then the boss can fire him with cause.
Good point. I've never worked at a place that had company security so I didn't think of it from that angle.