Sex and Friend feelings

A mutual friend of ours messaged him this week about hooking up and he's planning on getting a hotel room & loosing his virginity to her this weekend. They've established its just hooking up and that she's going to teach him about sex, that's all.

Now all of a sudden these feelings are welling up in me and I don't know what to do. I can't tell if I like him or if it's just this circumstance I'm uncomfortable with. I don't like thinking about them having sex, it makes me feel like I'm loosing him for some reason?  I'm confused and jealous and I don't know why. What do I do? How can I be happy for him if this is what he really wants?