The rear compartment of the Beast is I believe separated from the front by likely bullet proof glass. The POTUS would never ride in the front. This story is preposterous.
That is true, but the glass can be lowered. Supposedly, the only control for that is located in the rear section. Regardless, as you say, the claim in preposterous.
I have heard (unconfirmed) that this witness was trying to get a job at Mar-a-Lago after Trump left office, and Trump refused to hire her. She may be seeking revenge.