Uh, yeah, this.

I'd take the "I have to bring my own food because of cross-contamination purposes; please don't take this as an offence against your cooking" route, because a) even if they think it's completely clean, there's really no guarantee, b) this is pretty short notice for a Christmas event and kind of hard on everyone else.

Could have been handled better on all angles, though - from Barb, LW, and Amy all.