This is commonly known as the "Love Contract" approach. Essentially, any relationship between two people that could have a negative effect on the company if things sour, or if one party is able to improperly influence the other would fall under the policy.
Worse than supervisor and subordinate, because there is no HR or boss stopping the company owner, if things go wrong. Likely you will treat her differently than the others such as giving her information that she in her current position should not have, refusing to see her performance problems, etc. Is this age-old adage becoming extinct? How could THAT go wrong, right? Konerak, that is true, I just used the pronouns for the most common scenario. However, I'm curious that what exactly made you think that I would cause disturbance and unprofessional behaviour? I would change the can appear to will appear.