Welcome to the forums!
I am troubled that he slept with her before you were OK with it. I am also troubled that he 'rewarded' her after she was rude to you. Other than that, your start is pretty much the way a lot of people here started with slightly uneasy beginnings.
I am a bit unclear what you and he want from this point forward. He says that they are just sexual and friendly, but that in no way could they be in a relationship. But sleeping with someone and being friends IS in a relationship. Or is he saying he is breaking it off with her after this fling?
One thing that I have not heard you discuss is what are the boundaries and agreements for your relationship. I would suggest that these get discussed carefully. These agreements are not 'rules'. The thread below discusses this.
Newbie - what rules do you suggest?
You may wish to read post in the forum. This is an index of internal and external links I found useful.
Index to some threads from new to poly category.
You may wish to write back and give us more details, and more specific questions. Your current post lacks the details that our posters can sink their teeth into.
Warm regards, Rick.