No one is going to flame you for thinking poly is not for you -- we see enough stories here from people who couldn't make it work to know that it's truly *not* for everyone. And even if you *could* make it work, you're under no obligation to... while we could tell you all about the ways in which it can be great, it's also a lot of extra emotional work and no one should be forced to take that on if they know they can have an emotionally satisfying relationship in the way they're used to and want that instead.
If you and your husband are able to move on from this, but it still seems like it's something he needs (which I would in no way be sure of, since It sounds like this woman was really manipulating him... but it could be, nonetheless) and you decide you want to at least consider it, there are lots of good resources out there for "mono/poly" couples. But none of them will tell you that it's easy or that it's for everyone.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.