Found out 12 Hours ago. What now?

[LiM]

I suppose you could meet and talk to her but I would still hesitate on counseling right now. It's highly unlikely that she has completely come to grips with the reality of what she's done. Until she has hit rock bottom and knows in her heart that she has "lost" you because of this, then she won't find true remorse within herself. If you let her back too easily/quickly, then you will in effect be sweeping this under the rug and the chances of it happening again are VERY high because really, what did it cost her? Nothing. She knows she can go out and do whatever she wants and there will be no repercussions because you will simply take her right back.
Dont be a door mat. Make her work to earn her place back in the M.
You have to ask yourself, why is she coming around so quickly? I guarantee you that she hasn't found remorse yet. So what happened. Can you find out if OM went back home? Did he dump her and now she's going back to plan B (You)? If he did dump her, that's great because now you have a chance to start repairing the M but you can't just allow her to come right back.