(airport codes added by me)
The men started the flight at French Valley Airport (F70) and were headed to Borrego Springs (L08) but decided to land in Warner Springs because they determined they were losing daylight and visibility.
Warner Springs is a paved private strip used primarily for sailplanes and sailplane instruction. It’s in a valley at around 2900’ in elevation and unlit.
If the plane crashed at just before 5:30 pm, then they should never have left French Valley. It’s not very far between the two and the sun would have been very low on the horizon when they took off. Sunset down here is around 5pm right now.