Do you question your own worth? Or simpler said, how is your self-esteem?
Are you honest towards yourself and your true feelings?
If you believe in yourself and your own actions as being just (enough), then you don't have to compare your life to that of another and therefore you shouldn't feel the weight of jealousy.
If you doubt yourself or are dishonest however (even for a small part), chances are that the jealousy is a failsafe reaction as the other girl can be a risk towards your currently "stable" situation.
If he eventually does like her and the feeling is mutual, they might end up getting a relationship, meaning things will change and you will loose both the "benefits" aswell as how your friendship with the other girl currently is. And since you have no stereotypical relationship you can't blame anyone for anything but yourself. So don't blame yourself, ghehe.
I've had jealousy problems for quite some time in the past (in such a way that my best friend walked away with the girl i loved and the only person to blame was me; yet we all are still close friends nowadays), and i do admit it is human, and yes our society thrives upon it as it being one of our most basic foundations, but if you ask me its nothing but a waste of time, energy and money and gives humanity a common weak spot to be exploited on.
Eventually i tossed away my jealousy, stopped making unrealistic expectations and stopped comparing my own choices with those arround me and i'm feeling much more relaxed and happy then i've ever felt before!
I hope things eventually work out for you, i wouldn't be a happy camper either if i had to loose something i enjoyed without having any influence on the outcome.