First, confession and honest with him about how this all came up, how you feel, what you want and why.
Second, study up on poly.
(one and two may be interchangable, but both need to occur before proceeding)
Third, discuss poly with him if it still seems pertinent.
Fourth, discuss with her if it's still an option after 1-3.
Fifth, discuss between the three of you what the "boundaries" would be, including terms for protecting her from being out on her ass and homeless if the relationship "breaks up".
"Love As Thou Wilt"