Okay, so beneficiary has a pending petition with USCIS - the fact that someone filed a frivolous petition does not mean it will get approved by USCIS. We already told you if this person came into the US on a K1 visa and marriage never happened, there’s no legal path forward for them in the US. Being scheduled for a bio does not mean their stay continues to be a legal one from Nov. 13th, and that you should stop stressing yourself over their plans of finding some other means, albeit illegal, of remaining in the US.
I don’t think we’re getting the entire story here though: I’m curious to know how you came about this notice since you’re a ‘third party’ in all this. Your post also says “the sponsor received this mail ... regarding the application petition filed by the beneficiary for a biometrics ...” Where did the sponsor get the bio notice from? USCIS clearly didn’t send it to the sponsor, the notice would have been addressed and sent to the person scheduled for the bio appointment - the K1 beneficiary in this case.