Something went wrong somewhere and you need to reflect on this and provide answers. Try and find out where things went wrong between you and your husband. It might be true that they are involved but I advise you to talk to him about this.
If you cannot face him then there are many ways to communicate such as writing a note. Talking may help to square all this before it goes too far. But she has to go because keeping her will only bring more problems.
You can ask your maid to leave without necessarily raising heat. However, before you do that, be sure you got the correct information to guide your action. Have you asked yourself whether there could be a problem with you or your man to make him resort to her? Sending her away and employing another one is not a solution. You have to identify the root cause and address it.
Call a spade a spade here and share your thoughts with him. Otherwise, there is a chance that she could easily become your co-wife so act now or else your marriage is doomed.
A man is naturally attracted to any woman who performs wifely duties. You could have transferred the duties to her and consequently she took more than she deserved.
There are several things playing out here; one, is that something seems to have drastically changed in your marriage; two, at some point you suspected he was having an affair; three, a third party once warned you about the possibility of an illicit relationship between him and your house help and four, even when confronted with some evidence, you are still not convinced about this.
Wawu Gist 