I was recently asked to advise on a unique situation.  A local employer (ABC Company) recent fired an employee for stealing supplies and other dishonest activity.  Since then, more potential criminal activity has transpired between the ex-employee and the ABC Company and is under investigation.

On top of that, the employer has become aware that the previous employee has been recently hired to work at another larger company in the same industry.  Due to the nature of their businesses, this employees continued action has the potential to harm and/or affect others. 

Although the company never contacted ABC Company for a reference, and obviously failed to complete any background checks, ABC Company feels that the ethical thing to do would be to contact the new company and warn them. 

Obligation or not should they issue their warning?