My opinion is that deleting the thing is a very dangerous move. But it depends, if it is running on your own work computer/session, and it is like cleaning up

My opinion is that deleting the thing is a very dangerous move. But it depends, if it is running on your own work computer/session, and it is like cleaning up you data when leaving might be ok. As a real answer, we would need to know what are the terms of the contract of this person. His job description should be clearly stated inside and we would know if there is a kind of right/ copyright assignment to the company. If it is not the case, and it was not in his job description , it is easy to argue that the employee legally kept the copy right and rights on the software. So he could legally tell the employer that it is not allowed to use the software except buying it. The only gray area is if it was done during work hours with the employer willing fully agreed to have the employee spend some time working on that.