What is the best thing to do if now she is saying she is going to sue for adultery?

You can be sued for Alienation of Affection or Criminal Conversation if you engaged in a romantic or sexual relationship with the Husband prior to his date of separation. If your romantic relationship began after date of separation, then the law says you cannot be sued for either of the two claims. Wife knowing about the relationship or even condoning it, if it occurred before the date of separation, is not a defense. It can be used to convince a jury that the damages should be little or nothing, but it can't usually be used to have the suit dismissed. You need to consult an attorney. The specific facts are very important in cases like yours.