The main thing was because he hadn't expected Owen's behavior to be this appalling; that the man could stab him in the back.
Owen also thought Li was repulsive; he hadn't actually wanted to oppose Li but had wanted to strike it rich together with him.
He discovered later that Li Du had stopped selling him the gemstones he had found, and had no idea to whom Li Du had sold them to. He was, however, more concerned with having lost the chance to make any more money.
In any case, this was not the main reason for Owen to report him. The main reason was that after Hughes and Surgi had been arrested, they had dragged Owen into it.
Although he had the idea to, Owen was not directly involved in the illegal use of machines to destroy the National Park. This guy was very smart, and had conveniently disentangled himself from the matter when he was questioned by the police and the court.