Knowing,understanding and accepting that ones SO is polyamorous are completely different. I KNOW and UNDERSTAND my bf is poly but I think full acceptance is a way off,and I'm on a journey of figuring out whether it is the right path for me. I do get on well with his secondary and I accept his relationship with her because I TRUST her and her motives. His previous girlfriend couldn't deal with the polyamory at all and that is why they broke up,she was mono wired,pure and simple.
Just my feeling but your wife needs to figure out HOW it would be safe for her to bring your new friend into the fold. If she is never going to be ready,then you may have to make a hard decision.