Is it usual for a first officer not to know how to taxi?
After reading that article carefully, here's what appeared to have happened:


Pilot collapses
Passenger-with-pilots-license comes to flight deck (with both elitist and sexist chip on his shoulder)
First officer moves to left seat, PwPL takes right seat, runs the radios and checklists, maybe drops the gear.


First Officer is a right-seat qualification, on 737s (and many others) the steering handle is on the left side.

So it's quite possible that the FO has never steered a 737 on the ground. It's quite IMpossible that the FO has never taxied an aircraft of any kind before. And there's a huge difference between driving a Gulfstream around and driving a 130-passenger jet around.

If you don't take a corner just right, the main gear ends up in the ditch, the wing hits the ground, the engine inhales grass, gravel and maybe the taxiway sign and a couple of gophers. Trashing a Ferrari is cheaper.

The FO moving to the left seat and flying the plane was the correct action - the B-1 pilot was no more qualified to fly a 737 than I am. But radios are radios, and even the greenest flight attendant can read a checklist.