Okay, slow down, breathe. First, you are going to want to talk to Ben. There's some issues that are a little more pressing. Try some Non Violent Communication to help it go smoothly, accusations will start everyone on the wrong foot. Let him know that you want him to be able to be honest with you and you are sorry if maybe you weren't as open to hearing him as you should have been. Let him know that you heard him talking about this mutual friend and his interest but perhaps you weren't really listening but that you are listening now. He might have felt that he tried to talk to you before and it didn't go anywhere and so needed to hide things. Not a good enough excuse of course, but it sounds like it could be a communication issue to nip in the bud.
Now, breathing? Calmer? Then re-read what you wrote about this friend. Does it sound like you are thinking of her as person with her own thoughts and feelings or a toy for the two of you? You are willing for her to have a relationship with Ben but ONLY if she has an equal one with you too. Why are you deciding for her what her relationships should look like? Or how she will make YOUR lives easier, or how she might as well move to you since she has nothing else? It's not meaning to I'm sure, but it comes across as incredibly insensitive. She is not a puppy! She is not a child you are looking to adopt and put rules and strictures on so she can grow up a certain way. She's a person. Before jumping the gun into telling her what kind of relationship she can and can not have, what kind of life, and how she is going to fit into YOUR family, deal first with Ben and then consider talking to them both together.
If that doesnt' help, try reversing things and see how you feel. Ben comes to you and says he's super interested in her and she's interested in him but ONLY if you play ball. So now two people are telling you to be with the man you are with you MUST have a relationship with a woman you may never have considered before. Oh and you must love them both equally, make sure neither gets jealous of the other or god forbid you! Also, he will be happiest if you play 'second wife' at some point and make sure you take the pressure off of them with their work. OH! And she just got an awesome high paying job so you are moving there. Would you really be feeling the love there and want to jump that opportunity?
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year