Jack would be here anyway, even if he weren't involved in a certain secret celebration to come later in the day. Chuck is his friend, and he wants to support her efforts at purging the bad memories this place holds for him and Ianto and so many others here.
So he's here, being his usual affable, sociable self, mingling away, enjoying the champagne, flirting outrageously, and if the occasional shadow passes behind his eyes as his gaze falls on a certain patch of floor, who could blame him? It soon changes to a smile whenever he catches sight of Chuck on her happy day.