Hey there Needs,
Ok, lets see if this can clear the air and get you and your wife talking HONESTLY - about relationships and poly.
Her behavior regarding her BFF is really no different than any other form of jealousy we see. This is HER friend/BFF. There's the same old ownership problem surfacing here in that she feels she will 'lose' something if you and the bff become close. It's no different than the feared loss of a husband/wife/lover etc.
Wake up call #1:
She doesn't OWN her bff. The bff is a person with all rights & responsibilities. In poly - what's good for the goose is good for the gander. I suggest you start there in the discussion. The bff seems to be attracted to you (says you) and it's as much her call to make as anyones whether this moves forward. In all fairness, your wife needs to either step up and be TRULY poly - and accept same from you and her friend...and anyone else. Or abandon it herself ! Take your pick.
Ironically, this could be an enviable situation. In reality, there should be less opportunity for fear & angst of another lover for you because she already knows her ! Everyone is not as lucky and has to go through the whole getting to know/building trust scenario.
Don't bail just because of pressure - work it out. It's workable.