I'm guessing that after the initial screaming & yelling, she threw him out of the house immediately. Her comment in who are you pretty much hinted at that:
CATH: What's to talk about? You cheated on me. I caught you. I've got a new life. (basically saying "you cheated, you're out of here. end of story")
As for who she'd go to? Kind of a tough question. I'm guessing it wouldn't have been her mother. I can't see Lily being happy with Catherine's choice of Eddie. It could have been Sam. She might have wanted his strength to lean on. I'm guessing she would have gone to Gil - mainly for the quiet comfort he would probably give, but I'm not sure if that would have been right away or not. The question would be - how long after Catherine found Eddie cheating did Gil tell her that he knew what Eddie was doing?
This is why it would have been nice if they hadn't killed off Eddie so soon. With him gone, we really don't hear any more backstory into their marriage or separation.