I believe there have been enough of these kind of incidents where people have been hauled up for making inappropriate remarks to warn you about this. I seem to recall cases at LHR where Customs have marched people off for making so called "jokes" about having a gun/bomb in their possession. In this heightened level of security it seems especially daft to make that remark in the US.
Having said that, the possible penalty she may receive does seem a little harsh. Warning and caution perhaps? May be they want to make an example of her to stop others doing this.