It's unfortunate ... he's totally screwed himself over, because you seem like someone who is willing and able to put the work into having a poly relationship. Unlike your husband, who only wants to reap the "rewards" (i.e. to him, more sex) and have none of the work.
Talk to her. I don't know that this will actually solve anything, but if you leave the bastard, you should have the comfort of knowing you tried every available option. It's not you, truly.