Afraid my friend will leave me for his girlfriend

I am a girl and best friend is a boy. He like me for a really long time and I didn't know if I like him back so I kept pushing him away. We still are really close but this past week he had sex with some girl. He now wants to date her I support it in my head but my gut is telling me to be jealous. I don't understand why I feel this way. I am scared this girl is going to make him stop being my friend because he used to like me. I am just worried about loosing my friendship. Should I tell him this or see how their relationship plays out? I don't want him to think I don't like her don't trust his judgement what do I do?