Continue to do it as you did it last night -- you handled it well. Don't rescue (had you heard her behind a closed door, for example, I would have encouraged you to let her sob for 15-30 minutes, and then go in and do what you did and say what you said), but do show compassion.
It's a really fine line, and a difficult dance, but you're handling it well.
That's the good news. The bad news is, her sitting on your lap was likely all for show, and her sobbing -- at least for now -- is likely about HER, and not you. I've always said there are stages of remorse, and they progress from:
1. I'm sorry I got caught. 2. I'm sorry this is hurting ME 3. I'm sorry this is hurting OUR FAMILY 4. I'm sorry this is hurting YOU 5. I'm ready to own my part of this, and do the hard work necessary to see why I did this, and take steps to make sure it doesn't happen again.
I suspect that your wife is somewhere between Stage 2 and Stage 3 (your son's birthday party may have been the catalyst for taking her from Stage 2 to Stage 3).
Now, caveat: This is all MY theory, and I don't have any research to back it up. But I suspect that if you researched interpersonal behavior in general, you'd see some sort of similar progression.
Don't smother her today. In fact, create a little distance if you can. But maybe some some nice AOS at the end of the day to remind her of your loving presence (I used to like to cover up my wife with a blanket after she'd fall asleep on the couch, and softly kiss her forehead). But let her come to you if she wants to talk, and if she DOES, keep up with the morally-neutral validating of "this is hard on all of us." Draw a boundary and do NOT let her say anything to you like "I miss OM" or "you ruined my fun" or anything else that insults your integrity.
