'About a Boy': Dax Shepard visits as 'Parenthood' character Crosby Braverman
In the episode, Will has his buddies over for a poker night and Crosby has to bring new baby Aida because the babysitter is sick with food poisoning. It's just the cherry on top of the we're-so-old-and-can't-party-anymore cake, much to Will's consternation.
Fiona ends up getting invited because it turns out she's quite the card sharp, which really livens things up after Aida poops on the poker table and Crosby has to leave. Darn apricots.
But of course Fiona brings with her both hummus/yoga and shots/Oxford Stud, because she's a woman of many facets. The guys love her, and Will can't take it, so he snaps at her that her son is her only friend and she quietly excuses herself, which brings the party to a screeching halt.
In the end, Will and Fiona make up because Will has to drive a drunk Fiona to go get Marcus from his first sleepover (where he is failing miserably at having a good time, poor Marcus). Will talks Marcus into going back to the sleepover and all is right with the world.