I have been in the shoes of your wife. Nearly exactly in hers kind of in regard to this situation. I was sure that I had developed feelings for my long time friend, but as our relationship up to that point was purely long distance and I didn't even really know how he looked liked and if I would feel a real physical reaction towards him, we met to make sure of it. There were some strict rules set up for that meeting and we mainly managed to keep it ethically (as there was a real strong connection between us).
So meeting would be my advice as well, but as you are already in the picture (my husband wasn't at that point in time back then) all of you should talk about what is acceptable in terms of 'finding out' about any attraction there might be and what not. They should seek each others company on a every day like basis and see what comes out of it.
One word of warning in general though: You and she seem to be convinced that this will be something that will never be a topic again. But you yourself already classified her as poly/open. There will always be some kind of way for her to develop feelings. Whatever the precautionary measurements may be the two of you may take, there is always a possibility. I have been absolutely happy in my marriage and still, it happened. I am still focused on my husband, but there is another man right next to him. My ability of building a life with a special partner just became widened. And I as well, was convinced that this could never happen.
Wishing you luck and some peace of mind from time to time.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.