Rather than looking at it from the view of an employee or founder with power, I am attempting to view it through the lens of a good relationship between two people, with this being mainly defined through the golden rule - treating others as one wants to be treated.

It would seem to me that if I hadn't chosen to train and work with this person, he probably wouldn't have chosen this exact product to release out of all the possibilities in the world.

Let's say that he had an innate inclination for this exact product and would have done it even if he hadn't worked for me. Even then, he wouldn't have been privvy to the various and more detailed information that you are exposed to as an insider.

Going forward, I would be wary of any information I share with this person, given that it could work against me. This would definitely harm our relationship, and I would wish that this person would not do this to me.

If this were a fishing village, and the main thing to do around here was fishing, I would understand. There's not much choice there. But given that there are many different things this person could have worked on besides my exact product, my reaction would be, 'really man?'