Hullo and welcome!
There is only one cure for this situation - meet her. ASAP.
There is a very great likelihood that you are worried in vain, that they are totally comfy with poly, have done this for a while, and she truly is understanding of NRE and happy that her mate has met someone new and exciting.
Why do you feel you should have backed away immediately after learning he is poly? Do some reading around - there are plenty of people not only happy with their partners having OSOs but actually actively advocating they get one!
I know one couple where the wife wrote a letter to her hubby's gf who was battling with the very same issues you are, feeling like she was 'invading' their relationship. Maybe try writing a letter to the wife, explaining your concerns and feelings of insecurity, and then arranging to meet her?
Me: bi female in my twenties