I as thinking about all that stuff myself.. I wouldn't want her to start the conversation so it doesn't look like things were decided behind his back, besides, she shouldn't be stuck in the middle.
The best thing I thought of is for my those other friends maybe to bring it up... But My friends (the one from that poly relationship) says I should just wait and he might bring it up himself.
The other day I told him I told him I was going out with her to buy some stuff and just hang out and drink some beer. He as cool with that.. When I came back (she got up to the apartment for a second too) e got pissed like I didn't tell him or something. It was so weird and stupid and upsetting. Day after that he told me he was sorry and being stupid and said he'd tell her that as well.
It feels like when I take one step forward, I go back two steps :\