This was absolutely unsafe and illegal. Regardless of whether they were conducting a flight for friends or not, they were operating an aircraft that requires two pilots at all times (per the FAA) with just one pilot (and a student pilot).
That's unsafe and illegal, even if the plane was otherwise empty. Even worse with passengers.
This has existed for quite a while. The SR-71 (introduced in 1966) had an astro-inertial navigation system that tracked stars to reduce error in a standard INS.
Using spectrum filters, was able to track stars day and night (and I imagine potentially in bad weather).
However, transport category aircraft (among other issues) have swept wings, which are far less stable in a slip - so slips are neither taught nor recommended for jets.