Do you have evidence to suggest that shes having an A? If you do I think that I would simply say that you know whats going on because of A,B and C and you dont appreciate her direspecting you and your M like that. Or mention that she will lose her benefits if it comes out that she is involved with someone else.
I have a feeling that if you straight out ask if there is someone else, she will lie. They always lie. Or at least tell less than the whole truth, and you deserve to know the truth.
I am trying to think of a way to tell her that you know, or to sort of get her to confess in a way that wont cause a big fat fight. What about you telling her that it would be your worst fear for her to leave you for someone else, and you are so scared that thats whats happening, because(whatever your concerns are, intercepted emails or phone calls, whatever) and that you, and your M deserve for things to be honest.
Hmm, maybe a little too victim-y... Something like that though.
I guess I gave the wrong finger to the wrong man...
