No there is no law. Just plain human decency.
If we end up living in a society where human life is so meaningless it is a troubling world. We hope the best for anyone in our sphere - sports figures, work colleagues, family members, friends and neighbours.
For me personally it would change my impression of KJ if I knew him personally, and he did admit to not reporting the incident for days or weeks.
I am part of the society that states that our social contract requires that crimes be exposed. Turning a blind eye away from that type of stuff doesn't work for me. Especially with all the resources and protection that a person like KJ has being an NFL player.