If this is looking like it might blow up sometime soon you might want to talk with B about taking a break until he works things out (whichever way they are going to work out) with the live in partner. Sounds like there's likely going to be a lot of shit flying and I'm afraid if you're too close a lot of it is going to be aimed right at you.
B and the live in partner need to work a lot of stuff out. It could be that she's never going to be comfortable with anyone else being in B's life. It's then B's decision to stay with her or to move on to a new life.
Everything will be ok in the end. If it's not ok it's not the end.