I'm in the same situation ice however I'm the guy who caused the surprise nonmonogamy. I feel bad for my lovers husband and feel selfish but I think everything about love is selfish its just a matter of who is being selfish and in what capacity. When something like this happens you need to make a tough decision and lie in the bed you made for yourself.
a. deny your spouse a second lover under threat of divorce and separation, give her the ultimatum and make her choose, this will still cause resentment for both of you. Hers because she is being denied, yours because she had the nerve to bring it into your marriage. Not to mention the paranoia this will create because you can never be sure if she just decides to have an affair instead.
b. Allow the second lover for her happiness, and attempt to come to grips with it and learn from it. This creates resentment only for you but maybe you will learn to lose that resentment.