The first thing I would ask, are some questions for further clarifications. As her husband seems to be really demanding and inconsiderate. I don't know if you left that out, but is he working on this? Does he get how unfair the situation would be? It's one thing, if someone just doesn't want to have other partners or if he is told to not have other partners while his 'one and only' is doing as he pleases.
Why does she want to come to terms with this? Because he would end the relationship if she wouldn't? Because she regards his request as reasonable? Does she hope that things will change after some time? Does she even have the desire to form additional relationships? Or is this about theoretically having the opportunity. And is this about relationships from his side or does he want some physical outlet? Would it make a difference for her?
Well, I have had my share of this problem for sure. My thoughts seem to have been similar to her husband's. (If I interpret the situation right.) When things were really fresh and I still confused, I couldn't imagine a case, where I would have been in the shoes of my partner(s). It was the first entry on my blog with some real poly content.
Nowadays I have come to terms with this. Theoretically at least, I don't know how it would be in a practical situation. I guess, I would have some troubles coping at first. But I don't feel that uncomfortable thinking about a case, where one of my partners may be with another partner as well.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.